CO₂ sensitivity in the airways and the general skin surface of the bullfrog under urethane anesthesia or without anesthesia was investigated. Pressure in the buccal cavity as well as blood pressure in the sciatic artery were measured with a differential or a straingauge transducer. Air containing 2-14% CO₂ was introduced into the regions as given below.(1) The nose and the body surface, both regions were separated from each other and independently exposed to CO₂.(2) The larynx-lungs, the buccal cavity-lungs, the naso-buccal cavity separated from the airways at the glottis, and the internal or external nares, respectively. respectively. By analyzing the CO₂ sensitivity of the respective regions mentione above, both the nasal mucosa and the skin surface were found to be responsible for the respiratory inhibition by CO₂. Sectioning both the olfactory and the trigeminal nerves abolished the CO₂-induced inhibition mediated by the nasal mucosa and electrical stimulation of the proximal cut end of these nerves inhibited respiration. These findings suggested the existence of afferent reflex pathways from the nasal mucosa by these cranial nerves. Significance of this CO₂-induced reflex was discussed.

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The electrodermal reflex (EDR) was recorded from the paws of acute spinal cats by a DC potential recording method. The EDR was produced either by pinching the skin in various areas of the trunk, or by electrical stimulation of cutaneous afferents of various spinal segments. It was found that not only excitatory but also inhibitory EDRs were produced and that occurrence of the excitatory and inhibitory EDRs depended on both the segmental position and the laterality of the stimulated area. Cutaneous stimulation generally produced excitatory EDRs more frequently than inhibitory ones, except when afferent inputs en ered the spinal cord at or close to the segments of sudomotor outflow; in the latter case inhibitory EDRs were comparatively frequent and strong. Maximal inhibitory EDRs were elicited by stimulation of the group II afferent fibers of the cutaneous nerves only, whereas maximal excitatory EDRs were elicited after stimulation of the group II, III and IV afferent fibers of cutaneous origin.

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The effect and mechanism of volume loading (VL) on the baroreflex sensitivity, in terms of the changes in arterial pressure (AP) and heart rate (HR) to a given change in carotid sinus pressure (CSP), were studied in rabbits anesthetized with chloralose and urethan. The carotid sinuses were vascularly isolated in order to regulate the CSP independently from the systemic arterial pressure. VL was accomplished by intravenous infusion of dextran solution or whole blood to raise the right atrial pressure from 0.4 to 10 cm water. When the CSP was lowered from 90 to 40 mmHg, VL reduced the baroreflex pressor response by 10.1 mmHg (42%) and 19.9 mmHg (46%) before and after aortic nerves section, respectively. The attenuation effect of VL on the carotid pressor response was abolished after cervical vagotomy. VL also reduced the extent of bradycardia and hypotension upon an elevation of CSP from 90 to 140 mmHg. Before aortic nerve section, the reduction by VL was 19.7 beats/min (41%) for the bradycardic response, and 15.1 mmHg (43%) for the depressor response. These attenuation effects of VL were absent after aortic denervation. The results suggest that VL alters the carotid baroreflex pressor response via the vagal afferents subserving the cardiopulmonary receptors, whereas it reduces the depressor and bradycardic responses through the aortic baroreceptor.

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The effects of N-ethylmaleimide (NEM), a sulfhydryl blocking reagent, on the membrane currents, action potentials and contractile tension in bullfrog atrial muscle were studied by using the double sucrose gap technique. In concentrations of 10^-3 to 10^-4 M, NEM led to a transient enhancement of twitch contraction followed by a late inhibition, while 10^-2 M NEM merely produced an inhibitory effect. The positive inotropism was accompanied by a prolongation of action potential and the negative inotropism, by a depression of action potential. Under voltage clamp conditions, 10^-3 M NEM reduced the fast inward current and increased the steady state outward and background currents. The slow inward current and the inward Ca current in Na-free conditions were transiently increased by 10^-3 M NEM with an enhancement of contractile tension. In voltage-tension relationships, NEM-induced augmentation of tension appeared in depolarizing pulses of 60-70 mV. The results indicate that NEM increases transiently the Ca conductance of the cell membrane. Preincubation with L-cysteine blocked the production of positive inotropic effect by NEM, suggesting that the effects of NEM are due to the blockade of protein sulfhydryl group in the atrial muscle.

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The effects of N-ethylmaleimide (NEM) on the tension components of bullfrog atrium were studied under voltage clamp conditions using the double sucrose gap technique. NEM at a dilution of 10^-3 M inhibited the I_Ca-independent tonic tension elicited by large and long depolarizing pulses (140 mV, 2 sec) time-dependently, whereas the drug produced an initial transient increase and a later decrease of the I_Ca-dependent phasic tension elicited by small and short depolarizing pulses (60 mV, 100 msec). The effect of NEM was facilitated when the atrial muscles were exposed to a solution having either a reduced Na or an increased Ca concentration. In the presence of Mn ions, 10^-3 M NEM did not evidently inhibit tonic tension, and it led to a slight transient increase in phasic tension. NEM at 10^-3 M, which is known to inhibit sarcolemmal Na, K-ATPase, enhanced both the phasic tension and the tonic tension in the presence of ouabain and when perfused with K-free solution. Even under these conditions 10^-3 M NEM caused an initial transient decrease of I_Ca-independent tonic tension. This suggests that NEM has a different action on the atrial muscle from those of cardiac glycosides such as ouabain.

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Caffeine contracture in transverse tubules-disrupted (Tdisrupted) muscle preparation and the effect of anomalous anions (SCN and NO₃) on the contracture in single twitch fibers or thin bundles consisting of 2 or 3 fibers of frog were investigated. The magnitude and the rate of rise of caffeine contracture tension were decreased by Tdisruption and the decrease was more marked at lower concentrations of caffeine. No seasonal difference between summer and winter frogs on the effect of T-disruption on caffeine contracture was observed. On the other hand, the magnitude and the rate of rise of caffeine contracture tension in the intact preparation were increased by anomalous anions, whereas the rate of spontaneous relaxation of the contracture was decreased. The effect of SCN on caffeine contracture appeared 5 min after the pre-exposure of muscle preparation to SCN-Ringer solution, with the maximal effect appearing after 10 min. The threshold curve of caffeine contracture was shifted toward the lower caffeine concentrations by anomalous anions and the extent of the shift in the curve was more pronounced with SCN than with NO₃. In contrast, the potentiating effect of these anions on the caffeine contracture completely disappeared in Tdisruptedmuscle preparation. On the basis of these results, the caffeine pathway to the sarcoplasmic reticulum, and the sites and mechanism of the action of anomalous anions on caffeine contracture are discussed.

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In a certain group (type 2 cells) of bullfrog sympathetic ganglion cells, an action potential exhibiting a triphasic after-potential was produced when the cells were activated by direct intracellular stimulation. This triphasic after-potential consisted of two different potential components, namely, a depolarizing response (DR) and an after-hyperpolarization. The amplitude of DR was increased by increasing the interval between stimuli. Large DRs exceeded the threshold of the cell membrane and produced repetitive firings of spike potentials. The DR was selectively depressed and eventually eliminated in a low-Ca solution. Eserine (10^-5 M) reversibly increased both the amplitude and duration of the DR, and d-tubocurarine reversibly depressed it.
These results indicate that the DR of type 2 cells is the EPSP mediated by the nicotinic action of acetylcholine released from preganglionic nerve terminals. Preganglionic nerve fibers innervating type 2 cells are activated through some kind of recurrent pathway formed between them. Recurrent activation of type 2 cells seems to be thus induced when the cells are activated by direct intracellular stimulation.

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The binding sites of Ca²⁺ entering the injured cardiac muscle cell was investigated by ⁴⁵Ca electron microscope autoradiography in guinea pig papillary muscle and in frog ventricular muscle. The muscle was injured in Ca-free Tyrode's solution, transfered into ⁴⁵Ca-Tyrode's solution and, after 30 min, fixed and cut into thin sections. An autoradiogram was taken after exposure (4-7 weeks) the thin film of the section was developed. Fine, or sometimes filamentous, silver grains produced by radiation with ⁴⁵Ca were frequently observed on the intercalated discs of injured cells where most of the nexal membranes were separated. The number of ⁴⁵Ca grains on the discs of injured cells waseight times more than that found on intact cells. The concentration of ⁴⁵ Ca grains estimated by the number of grains per unit area was 3.2 times higher in the disc region than that in the other cytoplasmic regions in the injured cells, while in the intact cells it was only 1/3 of that found in thelatter. The localization of ⁴⁵Ca grains along the disc of the injured cell was also examined and the fine grains were seen to be located on the separated nexal membrane or at the cytoplasmic side of the desmosome and fascia adherens. It is likely that Ca²⁺ binds with the nexal membrane and the resulting structural change, such as nexal separation, is indispensable for intercellular uncoupling or healing-over of cardiac muscles.

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The slow PIII response of the electroretinogram (ERG) was studied in the isolated, aspartate-treated bullfrog retina. The slow PIII response reflected mainly rod activity in its time course and spectral response curve, and the ratio of the peak amplitude of the fast PIII response to the slow one remained almost constant within a scotopic range when rhodopsin was kept at a constant concentration. Reducing Na⁺ or K⁺ concentration in the extracellular fluid or decreasing the rhodopsin content in the retina, however, caused different influences on the fast and slow PIII responses, and, in particular, the reduction in amplitude of the slow PIII response was more marked than that of the fast PIII response. Under these circumstances, the log of the amplitude of the slow PIII response was in linear proportion to the amplitude of the fast PIII response. In addition, the amplitude reduction of the fast PIII response was not accompanied by a decrease in peak latency but that of the slow PIII response was. An explanation on the production of the slow PIII response was attempted with reference to other results related to the slow PIII response, ionic mechanism of the electrical activity of photoreceptor cells and their Na-K pump activity.

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The mechanical activity of the smooth muscle of guinea pig alimentary tract changed during fetal and postnatal development. At the F₂₀(fetus, body weight 20 g) stage of the fetus, the antrum, small intestinal and colonic preparations showed no spontaneous activity. After this stage, spontaneous activity occurred but the patterns of contractile activity of various regions of the alimentary tract differed from those of the adult. However, at later fetal stages (F₅₀or later), the mechanical patterns of various regions, except the lower region of colon, were similar to adult patterns. An excitatory response to acetylcholine was observed in all regions at the F₂₀stage but catecholamines were not effective at this stage. Responses to catecholamines appeared later than those to acetylcholine but they were not constant. During development of the colon, the excitatory effect of adrenaline was significant and the effect was blocked by phenoxybenzamine, and isoprenaline inhibited the spontaneous activity. However, sympathetic nerve stimulation at F₇₀inhibited the mechanical activity and propranolol blocked the inhibitory action of sympathetic nerves. These results indicate the co-existence of α-excitatory and β-inhibitory receptors on the developing colonic smooth muscle cell membrane.

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The effects of changing the intracellular pH on the action potential duration and other electrophysiological properties were studied in squid giant axons perfused intracellularly with TEA (tetraethylammonium)-containing solutions and under Ca-Na bi-ionic conditions. The duration of the action potential plateau, produced by TEA, was markedly decreased with acidic intracellular solutions and increased with alkaline intracellular solutions. The normalized duration, d, was calculated by dividing the plateau duration by that of the standard intracellularpH of 7.3, and plotted against the intracellular pH. The curve could be expressed by the formula log [d/(D-d)]=n (pH-pK') where D is the saturated value of d, n a constant and pK' the value of pH at which dbecomes D/2. The values selected for D and n were 2.6 and 1.4, respectively. The pK' was found to be 7.5. Lowering of the extracellular pH to 6.2 only slightly changed the plateau duration. Voltage clamp analysis revealed that acidic intracellular solutions decreased the size of the inward current and the slope conductance which were measured at the late period of the depolarizing clamping pulse. Alkaline intracellular solutions increased the size of the inward current and the slope conductance measured at this late period. Intracellular perfusion with a low pH solution also shortened the duration of the Ca-Na bi-ionic action potential. It is argued that the remarkable generality of the pH effect on the plateau duration implies the existence of a common mechanism of formation of the plateau under widely differing experimental conditions.

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