The endothelium of blood vessels plays a crucial role in the regulation of blood flow by controlling mechanical functions of underlying vascular smooth muscle. The regulation by the endothelium of vascular smooth muscle relaxation and contraction is mainly achieved via the release of vasoactive substances upon stimulation with neurohumoural substances and physical stimuli. Nitric oxide (NO) and prostaglandin I₂ (prostacyclin, PGI₂) are representative endothelium-derived chemicals that exhibit powerful blood vessel relaxation. NO action involves activation of soluble guanylyl cyclase and PGI₂ action is initiated by the stimulation of a cell-surface receptor (IP receptor, IPR) that is coupled with G_s-protein-adenylyl cyclase cascade. Many studies on the mechanisms by which NO and PGI₂ elicit blood vessel relaxation have highlighted a role of the large conductance, Ca²⁺-activated K⁺ (MaxiK, BK_Ca) channel in smooth muscle as their common downstream effector. Furthermore, their molecular mechanisms have been unravelled to include new routes different from the conventionally approved intracellular pathways. MaxiK channel might also serve as a target for endothelium-derived hyperpolarizing factor (EDHF), the non-NO, non-PGI₂ endothelium-derived relaxing factor in some blood vessels. In this brief article, we review how MaxiK channel serves as an endothelium-vascular smooth muscle transducer to communicate the chemical signals generated in the endothelium to control blood vessel mechanical functions and discuss their molecular mechanisms.

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Smooth muscle contraction is mediated by Ca²⁺-dependent and Ca²⁺-independent pathways. The latter Ca²⁺-independent pathway, termed Ca²⁺ sensitization, is mainly regulated by a monomeric GTP binding protein RhoA and its downstream target Rho-kinase. Recent studies suggest a possible involvement of augmented RhoA/Rho-kinase signaling in the elevated smooth muscle contraction in several human diseases. An increased bronchial smooth muscle contractility, which might be a major cause of the airway hyperresponsiveness that is a characteristic feature of asthmatics, has also been reported in bronchial asthma. Here, we will discuss the role of RhoA/Rho-kinase-mediated Ca²⁺ sensitization of bronchial smooth muscle contraction in the pathogenesis of airway hyperresponsiveness. Agonist-induced Ca²⁺ sensitization is also inherent in bronchial smooth muscle. Since the Ca²⁺ sensitization is sensitive to a RhoA inactivator, C3 exoenzyme, and a Rho-kinase inhibitor, Y-27632, the RhoA/Rho-kinase pathway is involved in the signaling. It is of interest that the RhoA/Rho-kinase-mediated Ca²⁺ sensitization of bronchial smooth muscle contraction is markedly augmented in experimental asthma. Moreover, Y-27632 relaxes the bronchospasm induced by contractile agonists and antigens in vivo. Y-27632 also has an ability to inhibit airway hyperresponsiveness induced by antigen challenge. Thus, the RhoA/Rho-kinase pathway might be a potential target for the development of new treatments for asthma, especially in airway hyperresponsiveness.

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To investigate the effect of oral glucose intake on gastric motility, we measured gastric myoelectrical activity and gastric emptying on two test conditions: 1) glucose intake and 2) water intake in the same 10 healthy male volunteers (20 to 29 years old). Gastric motility was evaluated with cutaneous-recorded electrogastrography (EGG) for 30 min both on fasting and after glucose or water intake, while gastric emptying was measured using acetaminophen-absorption method. There were no significant changes in EGG dominant frequency after water intake, but the frequency increased significantly after glucose intake. A postprandial dip (i.e., a transient decrease in frequency immediately after the food intake) was observed in 3 subjects after water intake and in 8 subjects following glucose intake. The EGG power ratio was significantly larger after glucose than water intake, with delayed gastric emptying in the former case. These results suggest that glucose is one of the components responsible for postprandial gastric motility.

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We have compared the reactivity to carbachol and high potassium of circular smooth muscle isolated from segments of human colon which was freeze-stored in different preservative solutions for more than one month following surgical resection. Concentration-dependent contractions in response to carbachol were reduced in terms of both their sensitivity (pEC₅₀) and reactivity (Emax), depending on the preservative solutions used. Similar reduction of reactivity to 100 mM KCl was also observed. The best responsiveness was shown when the tissue was freeze-stored in SFM101. It is concluded that the freeze-storage of surgically excised human colon in SFM101 or phosphate buffer solution for more than one month provided the best preservation of smooth muscle function for in vitro pharmacological examination.

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The effects of endogenous and exogenous nitric oxide (NO) on electrical activity were investigated in circular smooth muscle preparations isolated from the guinea-pig stomach antrum. The actions of endogenous NO were evaluated from the effects of inhibition of NO synthesis by N^ω-nitro-L-arginine (nitroarginine), while those of exogenous NO were assessed from the effects of SIN-1, an NO donor. Antral circular smooth muscle generated slow potentials periodically at a frequency of about 1 cycle per min (cpm), and unitary potentials were also generated in a random fashion in the interval between slow potentials. Application of nitroarginine (10^-5 M) increased the frequency of slow potentials, with no significant alteration of the resting membrane potential and amplitude of slow potentials. Frequency analysis of unitary potentials revealed that nitroarginine also increased the spectral density at 0.01-1 Hz frequency. The refractory period for the generation of slow potentials evoked by depolarizing pulses was about 10 s, but was decreased to 6 s by nitroarginine. In the presence of nitroarginine, SIN-1 (10^-9-10^-7 M) reduced the amplitude and frequency of slow potentials: low concentrations (<10^-8 M) reduced only the frequency of slow potentials, while higher concentrations (10^-8-10^-7 M) reduced both the amplitude and frequency of slow potentials, in a concentration-dependent manner, before abolishing the slow potentials. The power spectrum of the unitary potentials indicated that SIN-1 (>10^-8 M) reduced the spectral density at 0.01-1 Hz frequency. The refractory period for the generation of slow potentials was increased again to about 10 s by SIN-1. Thus, the excitatory effects of nitroarginine could be antagonized by SIN-1, suggesting that the inhibitory effects of endogenous NO are comparable to those of exogenous NO produced by SIN-1. The results also suggested that the effects of NO on smooth muscle are insignificant and NO selectively inhibits the activity of intramuscular interstitial cells of Cajal (ICC-IM).

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Electrophysiological effects of endothelin-1 (ET-1) on circular smooth muscle of rat gastric antrum were investigated by using intracellular membrane potential recording techniques. ET-1 (10 nM) caused an initial hyperpolarization of the membrane which was followed by a sustained depolarization. ET-1 also increased the frequency but not the amplitude of slow waves. In the presence of the endothelin type A (ET_A) receptor antagonist, BQ123 (1 μM), ET-1 (10 nM) depolarized the membrane and increased the frequency of slow waves, but without the initial hyperpolarization. The selective endothelin type B (ET_B) receptor agonist, sarafotoxin S6c (10 nM), also depolarized the membrane and increased the frequency of slow waves. In the presence of the ET_B receptor antagonist, BQ788 (1 μM), ET-1 (10 nM) hyperpolarized the membrane. However, in the presence of BQ788, ET-1 caused neither the depolarization nor the increase in the frequency of the slow waves. The ET-1-induced hyperpolarization was completely abolished by apamin (0.1 μM). In the presence of apamin, ET-1 depolarized the membrane and increased the frequency of slow waves. The ET-1-induced depolarization was significantly attenuated by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS, 0.3 mM). The increase of the frequency by ET-1 was observed both in the presence and absence of DIDS. These results suggest that, ET-1 hyperpolarizes the membrane by the activation of Ca²⁺-activated K⁺ channels via ET_A receptors, and depolarizes the membrane by the activation of Ca²⁺-activated Cl^- channels via ET_B receptors. ET-1 also appears to increase the frequency of slow waves via ET_B receptors, however this mechanism would seem to be independent of membrane depolarization.

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Previous study has revealed that water-responsive afferent neurons in the superior laryngeal nerve induced inhibition of motility in the proximal and distal stomach using anaesthetized rats. These gastric responses might facilitate the reservoir function of the stomach. To confirm the gastric responses discovered in rats also occur in humans, we evaluated gastric myoelectrical activities in healthy volunteer subjects during fluid intake using electrogastrography. Before human experiments, we recorded the myoelectrical activities in rats to evaluate the response induced by the administration of water into the larynx. A large deflection in the gastric myoelectrical activities was observed just after the administration of water in anesthetized rats. A similar large deflection was also observed just after voluntary swallowing of 20 ml water in humans. The swallowing of saliva did not induce such response. We further observed the gastric response during reflex swallowing elicited by slow infusion of isotonic saline, water or 0.05 M citric acid on to the posterior tongue. Infusion of water and citric acid but not 0.15 M saline induced significant changes in mean relative ratio of the response. These electrogastrographic responses induced by the infusion of liquids strongly suggest that the gastric motor response facilitates reservoir function of the stomach during liquid intake in humans as well as in rats.

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