The nicotinic cholinergic transmission causing generation of the fast EPSP in bullfrog sympathetic ganglia is modulated by the action of transmitters or hormones other than ACh. In general, a synaptic transmission appears to be modulated in a variety of ways by the action of many kinds of transmitters or hormones in the vertebrate neural system. There are at least four different types of modulatory actions of these endogenous substances on a synaptic transmission; namely, 1) modulation of the amount of the transmitter released from presynaptic neurones, 2) modulation of the sensitivity of the receptors of postsynaptic neurones, 3) modulation of the resting membrane potential or conductance of postsynaptic neurones, and 4) modulation of the configuration of the action potential of postsynaptic neurones. In the present review, the experimental evidence supporting the modulatory actions of endogenous substances on the receptor sensitivity and the action potential of postsynaptic neurones was reviewed. The mechanisms underlying these two types of modulatory actions were also briefly discussed.

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When the concentration of external Ca²⁺ was reduced for 30sec in a single twitch muscle fiber of a frog, the peak tension of the initial component of biphasic 80mM K⁺ contractures was potentiated, whereas that of the secondary component was markedly inhibited, despite the fact that in the early stage it was potentiated usually in case of contractures in 60 and 40mM K⁺ but rarely in 80mM K⁺. These changes were not observed, however, in the presence of 0.5-1mM Mn²⁺, i.e., Mn²⁺ having been substituted for Ca²⁺. The foregoing result and the authors' previously reported data indicated the following. First, the concentrations of divalent cations having an equal effect in bringing about the peak tensions of both components are 3mM Mg²⁺, 0.7mM Mn²⁺, 0.5mM Ni²⁺, and 1.8mM Ca²⁺. Secondly, this sequence constitutes their increasing order of effectiveness on the time course of the secondary component. Meanwhile, a similar order was found to exist in another experiment concerning the effectiveness in inhibiting the inactivation of K contractures facilitated by lowering the concentration of external Ca²⁺. All these findings indicated that the divalent cations act on the activation processes of both components in a stabilizing manner, differing from the way in which they act on the inactivation process of the secondary component. The mechanisms in which the peak tensions of the initial and secondary components are inhibited in a low Ca²⁺ solution and the divalent cations act on both components are discussed. Finally, another experiment made in the absence and presence of Ca²⁺ revealed that the effect of high concentrations of Mn²⁺ on the initial component is different from that on the secondary component.

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Intracellular recording of the resting membrane potential was made from taste cells of the bullfrog by replacing the interstitial fluid surrounding the cells by various physiological saline solutions. The resting potential of the taste cell was -28±4mV (mean ± S.E.) after replacement of the interstitial fluid by normal Ringer solution. The resting potential was very much dependent on the interstitial K⁺ concentration ([K⁺]_o). Tenfold increase in [K⁺]_o decreased the resting potential by 18mV. Total removal of interstitial Na⁺ increased the resting potential by 40%. Ouabain in a concentration of 0.5mM decreased the resting potential by 36% possibly due to inhibition of the Na⁺-K⁺ exchange pump. Neither total removal nor fourfold increase of interstitial Ca²⁺ changed the resting potential. Complete elimination of interstitial Cl^- did not change the resting potential. The mean permeability ratio P_Na/P_K was calculated to be 0.41. It is concluded that the resting potential of a frog taste cell consists of ionic and metabolic components, and that the ionic component is due to the permeability of the cell membrane to K⁺ and Na⁺.

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The net fluid and protein movements between intravascular and interstitial space following blood withdrawal and retransfusion of 15% of circulating blood volume were measured in dogs using a continuous monitoring method of circulating blood volume. A significant (p<0.01) increase in transvascular fluid shift was observed after the start of blood withdrawal and a new equilibrium state was reached within 15 to 20min. Associated with the fluid shift, a significant increase in plasma protein of about 1g was observed. On the other hand, blood retransfusion caused significant (p<0.01) increases in the shift of fluid and protein from intravascular space to interstitial space. The magnitudes of the shift of fluid and protein were almost identical in both blood withdrawal and retransfusion. The Kedem-Katchalsky transport equation was applied to the results obtained in the present study in order to analyze the relative role of diffusion and convection and to estimate the reflection coefficient for protein. A significant (p<0.01) linear relationship was observed between fluid and protein movement. These results suggest that the convective process is involved in the shift of protein between intravascular and interstitial space observed after blood volume modification.

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In anesthetized dogs, intravenous injections of 30μg/kg of leucine- and methionine-enkephalin (L- and M-enk respectively) produced a significant reduction of mean blood pressure (MBP) and heart rate (HR). The peak decreases in MBP and HR occurred within 1min after injection and recoveries to pre-injection levels within 3min. The depressor responses to both L-enk and M-enk were abolished by intracis-ternal pretreatment with yohimbine (0.5mg/kg) but not with prazosin (0.1mg/kg). Pretreatment with either drug did not alter the bradycardia. All cardiovascular effects of both L-enk and M-enk were prevented by intravenous pretreatment with naloxone (2mg/kg). These results suggest that central α₂-adrenoceptors may participate in the central actions of enkephalins on blood pressure regulation. HR responses to enkephalins may be elicited by a different mechanism.

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In order to clarify the interaction between CO₂ diffusion and HCO₃^- shift in the red blood cell (RBC), HCO₃^- shift was measured by using a stopped flow method combined with fluorometry. When HCO₃^- entered the RBC, the intracellular P_CO2 increased, causing a secondary outflow of CO₂. Conversely, when HCO₃^- ions flowed out of the RBC, the resulting decrease of P_CO2 caused an inward CO₂ diffusion. The P_CO2 change caused by the inward HCO₃^- shift was about 3- to 4-fold that of the outward shift. During the respective in- and outward-shifts, the mean half times of the extracellular pH changes were 0.15 and 0.13sec. These were approximately twice as long as those of the primary CO₂ diffusion. The permeability of HCO₃^- across the RBC membrane was obtained by comparing the experimental extracellular pH curve with a numerical solution for CO₂ and HCO₃^- diffusions accompanied by the hydration and dehydration reactions. Thus the HCO₃^- permeability was determined to be 5×10^-4 and 7×10^-4cm/sec, in the in- and outward-HCO₃^- shifts, respectively. The influence of Cl^- concentration on HCO₃^- permeability was tested by reducing the initial Cl^- gradient across the RBC membrane. In a physiological Cl^- concentration range the HCO₃^- permeability was not affected by the Cl^- gradient.

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To reveal the CO₂ diffusion process into and out of the red blood cell (RBC), changes in P_CO2 following the HCO₃^- shift were examined. The RBC suspension was mixed at 37°C with saline solution having different HCO₃^- concentrations. In proportion to the intracellular HCO₃^- change caused by the HCO₃^- shift, the P_CO2 change was increased. The rate of change increased as the hematocrit and the initial intracellular HCO₃^- content decreased. For calculating the above P_CO2 change, a theoretical equation was derived from the Henderson-Hasselbalch equation and the relation between both the changes in buffer base and intracellular pH. The calculated P_CO2 change coincided well with the measured value, suggesting the validity of the theoretical equations.

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A computer program for a numerical solution of the CO₂ diffusion in the red blood cell including the HCO₃- shift was developed. Comparing the calculated extracellular P_CO2 curve with the experimental data on the primary and secondary CO₂ diffusions, the permeabilities of CO₂ and HCO₃- across the red cell membrane, η(CO₂) and η(HCO₃-), were determined as η(CO₂)=2.5×10^-6cm/(sec•Torr), Inward η(HCO₃^-) =5×10^-4cm/sec, Outward η(HCO₃^-)=7×10^-4cm/sec. The dependencies of the CO₂ diffusion rate on the hematocrit and initial intra- and extracellular HCO₃- concentrations were examined using the above permeabilities. The half-time of the extracellular P_CO2 change in a closed RBC suspension was obviously reduced as the hematocrit and initial extracellular HCO₃^- increased, while the effect of the initial intracellular HCO₃^- was not significant. The CO₂ diffusion in vivo was simulated using pulmonary and tissue capillary models. The half-time of the extracellular HCO₃^- change was about 0.15sec, while that of the intracellular HCO₃^- change ranged from 40 to 60msec.

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1. The effects of specific protein modifying reagents on the dose (Glu)-response (ΔG) relationship of the glutamate-hyperpolarizing (Glu-H) receptors in molluscan neurons (Onchidium verruculatum) were analyzed. 2. The effects could be classified into three types. (1) Type I, parallel shift of the dose-response curve towards higher concentration by modification of COO^- groups by N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ). The double reciprocal plots of the dose-response curves before and after the treatment indicated competitive inhibition type modification. (2) Type II, a decrease in the slope of the dose-response curve through modification of -SH by N ethylmaleimide and -NH₃ groups by trinitrobenzenesulfonic acid. The double reciprocal plots indicated non-competitive inhibition type modification. (3) The combined effects of Type I and Type II, after modification of arginyl residues with diacetyl trimer. The modification was most effective when applied during an activated state of receptors and channels by Glu. 3. EEDQ had an irreversible after-effect on Glu-H type Onchidium neurons to activate an additional Na⁺ permeability increase in Glu induced hyperpolarizing response. 4. Pretreatment of N acetylimidazole (NAI), Glu-H (including weak D) receptors in Helix aspersa produced no significant difference in Glu response. However, simultaneous application of NAT and Glu induced an additional Na⁺ permeability increase, probably by modification of tyrosyl residues. This indicates the greatest effectiveness of the NAI modification during the activated state of the receptors and the channels by Glu. 5. It is suggested that the Glu-H receptor protein possesses both negatively and positively charged residues, containing COO^- and arginyl⁺ in the receptive site, and that several amino acid resides (-SH, -NH, arginyl, and tyrosyl) act in the Glu-activated receptors and channels as their subsites. 6. The role of the subsites in the receptors and channels is discussed.

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While applying sinusoidal indentations of various frequencies and amplitudes to the receptive field of cutaneous mechanoreceptor afferent units in frogs, we measured latencies of spikes after the time point when sinusoidal stimulation crossed the neutral position from withdrawal to indentation phase. Frog type I (Ft I) and frog type II (Ft II) slowly adapting (SA) units produced spikes only at the indentation phase, and the threshold response phase (TRP), i.e.the phase of the first spike was for ca. 45°at a small amplitude of stimulation. At low frequencies of sinusoidal stimulations, the TRP of Ft II units advanced gradually with an increase in amplitude, conforming to the theoretical expectation as displacement-detectors. But the TRP of Ft I units did not show such an advance with an increase in stimulus amplitude. Rapidly adapting (RA) type I and type II units mostly produced phase-locked discharges at two different phases; the former discharged at ca. 0° and 180° (phase of the highest velocity) and the latter at ca. -90° and 90° (phase of the highest acceleration) at the smallest amplitude used. At a large amplitude, RA type II units showed advance in discharge phase like a theoretical acceleration-detector, but performance of RA type I units did not fit that of the theoretical velocity-detectors. Both types of SA units showed frequency-phase relations which roughly agreed with the performance of the theoretical displacement-detectors in the range below 5Hz, but RA units of either type did not follow the expectation as the theoretical velocity-detector or acceleration-detector.

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Intracellular K⁺ activity (a_Kⁱ) and transmembrane potential differences were simultaneously measured with double-barreled K⁺ selective microelectrodes in superfused canine submandibular glands at rest and during stimulation. Also intracellular K⁺ and Na+ concentrations in the same gland were determined by chemical analyses for comparison with the intracellular K⁺ activity data. The activity coefficient for intracellular K⁺ thus obtained was 0.67, indicating that virtually all K⁺ ions in the cells are in diffusible and osmotically active form. Under control conditions, measured values of the resting membrane potential (E_m), aKⁱ and K⁺ equilibrium potential (E_K) were -40.2±0.6mV (S.E., n=163), -88.5±1.4mM (S.E., n=163), -83mV respectively. Electrochemical potential differences for K⁺ across the basolateral membrane (Δμ_K/F) were approximately +43mV. Acetylcholine (ACh) induced an abrupt membrane hyperpolarization followed by a temporary fall of a_Kⁱ. The maximum rate of a_Kⁱ change in the cytoplasm during stimulated conditions was -35.5±0.9mM/min on the average (n=10). These results suggest that the membrane permeability increases to K⁺ upon stimulation. Possible changes in membrane permeabilities to (NaE_K) and Cl^- were also discussed.

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Properties of isolated single smooth muscle cells in which the surface membrane has been made highly permeable by saponin treatment are described. The single cells were isolated from guinea pig taenia caeci by digestion with collagenase and skinned in a relaxing solution by treatment with 7μg/ml saponin for 10min. The skinned single cells showed Ca²⁺ -regulated shortening in the presence of Mg-ATP, and the maximum degree of shortening was larger than that of the potassium-induced shortening observed in the intact isolated single cells. The half maximum shortening of the skinned single cells occurred at about 5×10^-8M Ca²⁺. The skinned single cells showed a significantly higher Ca²⁺ -sensitivity than the skinned fiber bundles. The shortening-pCa curve for the skinned single cells was unchanged by alteration of pH and ionic strength, but it was shifted to the left by increasing MgATP concentration Caffeine induced shortening in the skinned single cells preloaded with Ca²⁺, and this shortening was suppressed by procaine. The release of Ca²⁺ from the storage site could be produced and facilitated by Ca²⁺ itself when the skinned single cells were exposed to Ca²⁺ with a concentration of above 2×10^-8M and this release was suppressed by procaine. These results suggest that the Ca²⁺ -induced Ca²⁺ release mechanism may play a role in the regulation of the stored Ca²⁺ in this cell.

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The influence of varying the ionic strength of the bathing solution on the contraction of chemically skinned frog atrial muscle fibers was studied. The rate of tension development activated by calcium slows as the ionic strength is elevated. The size of caffeine contracture, however, was larger in the fiber preloaded with calcium at a higher ionic strength. There was a decrease in the maintained tension at 10^-6M Ca when a fiber was bathed in a high ionic strength solution. Returning to a normal ionic strength solution caused a transient tension increase. When the fiber was bathed in a low ionic strength solution, the maintained tension increased transiently to a high value and then declined to reach a plateau. The response was also observed in a solution of pCa 8. In the caffeine-treated fiber or in the fiber bathed in ATP free solution, although the maintained tension level was changed corresponding to the altered ionic strength, the transient responses were blocked. These responses were not much influenced by the kinds of salts used to change the ionic strength. When osmolarity of the medium was altered by sucrose, transient responses were not induced. The results could qualitatively explain the isometric tension change of an intact fiber of frog atrium bathed in a hyper- or hypotonic solution.

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Cation contents of various cardiac regions were determined in the isolated perfused bullfrog heart. The Ca content was much higher in the pacemaker tissues (sinus venosus and atrio-ventricular ring muscle) than in the atrial or ventricular muscles even with extracellular Ca deficiency. A possible relationship between stored Ca and spontaneous pacemaker activity is discussed.

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The current through a myocardial inward rectifier K⁺ channel was recorded by the patch clamp technique. The inactivation gate of the channel had open probability of nearly 1 at the potentials positive to EK and showed peculiar kinetics in the 1st open events after hyperpolarization from these potentials. Analysis of this kinetics suggested two open states in this gate.

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The differences between V_O2 at the 5th min in steady state exercise and V_O2 in incremental exercise were consecutively added from start of incremental exercise to a given time. These added values were defined as oxygen deficit. Anaerobic threshold was determined as the breakpoint of blood lactate during incremental exercise. The oxygen deficit at the anaerobic threshold in normoxia was almost identical to that in hypoxia.

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In the identified molluscan neuron, the presynaptic stimulation evokes a histaminergic inhibition of long duration (HILD), resulting from an increase in K conductance followed by a conductance-independent hyperpolarizing process. The later component of HILD was enhanced by cyclic AMP but depressed by cyclic GMP. However, neither agent alone altered the resting membrane potential nor the conductance.

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In rat thymocytes, the effect of neuraminidase on rosette formation with guinea pig erythrocytes was studied. In medullary thymocytes, the percentage of rosette formation increased significantly after neuraminidase treatment. Some parts of receptors for rosette formation in thymocytes might be masked by sialic acid in the course of maturation.

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