The Ca²⁺-dependent, reversible phosphorylation of the 20 kDa regulatory myosin light chain (MLC) plays a primary role in regulating the contraction of smooth muscle. However, it is well known that the Ca²⁺ signal is not the only factor which regulates such contraction, however, the alteration of the Ca²⁺ sensitivity in the contractile apparatus is also known to play an important role. The degree of MLC phosphorylation is determined by the balance of the activity between phosphorylation and dephosphorylation. Either the Ca²⁺-independent activation of MLC phosphorylation or the inhibition of MLC dephosphorylation causes a greater MLC phosphorylation for a given level of Ca²⁺ signal and thereby potentiates the myofilament Ca²⁺ sensitivity. The smooth muscle myosin light chain phosphatase (MLCP) consisting of three subunits was first isolated and cloned in the early '90s. The intensive investigation thereafter has uncovered the biochemical basis for regulating the activity of MLCP. The regulation of the MLCP activity is now considered to play a critical role in regulating the myofilament Ca²⁺ sensitivity. There are three major mechanisms in the regulation of MLCP; (1) the phosphorylation of a 110 kDa regulatory subunit of MLCP (2) the conformational change of the trimeric structure, and (3) the inhibition by a smooth muscle specific inhibitor protein, CPI-17. Furthermore, some kinases have been found to phosphorylate the MLC and activate the contraction of smooth muscle in a Ca²⁺-independent manner. Numerous protein kinases have been found to be involved in the regulation of MLC phosphorylation, and rho-kinase is one of the most frequently investigated kinases. The smooth muscle physiology is now asked to integrate the current understanding of the biochemical mechanisms and to clarify which kinases and/or proteins in the contractile apparatus play a physiological role in regulating the myofilament Ca²⁺ sensitivity and how such extracellular contractile stimulation modulates these mechanisms.

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The parasympathetic nervous system is responsible for maintaining normal intestinal and bladder function, contracting the smooth muscle by releasing the neurotransmitters acetylcholine (ACh) and ATP and relaxing sphincters by releasing nitric oxide. ACh is the main transmitter released and smooth muscle contraction is mediated via a mixed M2/M3 receptor population; M3 receptors acting via phospholipase C and M2 receptors acting via inhibition of adenylate cyclase. In ileal, colonic, gastric and bladder (detrusor) smooth muscle the density of M2 receptors is far greater than the density of M3 receptors, the M2:M3 ratio being 3:1 in most species including man. Despite the predominance of M2-receptors, direct contraction of intestinal and detrusor smooth muscle is mediated via the M3-receptor subtype and only this subtype is involved in contraction in vitro. Furthermore, knocking out the M3-receptor gene can have severe consequences on intestinal and bladder responses. In some tissues however M2-receptors may mediate an indirect "re-contraction" whereby a reduction in adenylate cyclase activity reverses the relaxation induced by β-adrenoceptor stimulation. Thus, intestinal and bladder responses to muscarinic agonists are slightly depressed in M2 receptor knockout mice. The role of receptor subtypes in disease is unclear, but an enhancement of M2 receptor mediated responses has been reported to occur in diabetes. Animal models suggest that M2 receptors may play a greater role in some situations such as in the denervated bladder and intestine. In human disease the mechanisms operating are not so clear. Detrusor sensitivity to muscarinic agonists is enhanced in the neurogenic overactive bladder, but there is controversy surrounding the role of M2 receptors and conflicting results have been reported. Thus, the main muscarinic receptor mediating contraction in normal smooth muscle is the M3 receptor, but M2 receptors are also present and possibly may have an enhanced role in disease.

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This study was performed to determine the effect of forced swimming on the vascular responsiveness of the rat superior mesenteric artery to phenylephrine, focusing on the involvement of locally produced substances. Repeated but not single sessions of forced swimming exercise reduced the vasoconstrictor potency of phenylephrine in the studied arteries, regardless of the presence of intact endothelium. No significant changes were observed in the maximal response to phenylephrine. Treatment with indomethacin (1 μM) did not affect the exercise-induced reduction in vascular responsiveness to phenylephrine. However, the reduction of vascular reactivity to phenylephrine due to repeated exercise was no longer observed after treatment with N^G-monomethyl-L-arginine (L-NMMA, 100 μM). The results suggest that repeated exercise reduces vasomotor responses to phenylephrine in rat superior mesenteric arteries through a non-endothelial nitric oxide (NO)-related mechanism.

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Intracellular recordings of electrical activity were made from circular smooth muscle cells in small segments of tissue isolated from the guinea-pig stomach antrum. Every cell that was impaled exhibited a rhythmic generation of slow potentials. Experiments were carried out to test the effects of three different concentrations (1, 10 and 100 nM) of phorbol 12, 13-dibutyrate (PDBu) on these slow potentials and on the responses produced by acetylcholine (ACh), in the presence of nifedipine and N^ω-nitro-L-arginine (nitroarginine), known inhibitors of L-type Ca-channels and nitric oxide synthase, respectively. The resting membrane potential was -62 ± 7 mV, while the frequency and amplitude of the slow potentials were 1.6 ± 0.1 cycle per min (cpm) and 33 ± 1 mV, respectively. Application of 1 nM PDBu increased the frequency of slow potentials, with no significant change in the membrane potential and amplitude of slow potentials. At a concentration of 100 nM, PDBu depolarized the membrane by about 6 mV, and either decreased the amplitude and frequency of the slow potentials or abolished them. The amplitude and frequency of the slow potentials were not significantly changed in the presence of 10 nM PDBu. In the presence of chelerythrine (1-2 μM), a known inhibitor of protein kinase C (PKC), the increase in frequency of slow potentials by 1 nM PDBu and depolarization produced by 100 nM PDBu were not elicited. The increase in frequency of slow potentials by 100 nM ACh was inhibited by PDBu, in a concentration-dependent manner, and ACh-responses were abolished in the presence of 100 nM PDBu. These results indicate that PDBu has dual actions on the spontaneous activity of antral circular muscle, with low concentrations increasing and high concentrations inhibiting the frequency of the slow potentials. The former may be produced by activation of protein kinase C (PKC). As the ACh-induced excitation of slow potentials is inhibited by PDBu, a possible causal relationship between the inhibition and over-activation of PKC is considered.

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Electrogastrograms (EGGs) were recorded in patients both before and after receiving proximal gastrectomy plus jejunal interposition (PGJI) or just after receiving total gastrectomy plus jejunal interposition (TGJI). Intraluminal pressure was also recorded in some postoperative patients. The EGG 3 cpm component (2.5-4.9 cpm) remained after PGJI, but subsequently decreased with a significant reduction in the preoperative to postoperative ratio of the 3 cpm components (P<0.05). The mean frequency of the 3 cpm components increased significantly after PGJI (P<0.05) and its instability factor increased. The EGG 10 cpm components became relatively dominant compared to other frequency components in 2 out of 8 of patients having PGJI but the mean amplitude of 10 cpm decreased. In TGJI patients, only the 10 cpm component was conspicuous in EGG as in the case of total gastrectomy and Roux en Y anastomosis procedures. The spectral frequencies of intraluminal pressure in the interposed jejunum were similar to the EGG of 10 cpm components both in the case of PGJI and TGJI patients. In conclusion, surface EGG could record the electrical activities of the interposed jejunum more easily in patients having had TGJI than in PGJI.

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