ABSTRACT. In the present study mechanism of inhibitory effects of capsaicin on the contractility of rabbit coronary artery were studied by measurement of isometric tension and intracellular Ca 2+ concentration. Capsaicin (1 µM to 30 µM) relaxed the coronary artery pre-contracted with prostaglandin (PG) F 2α (1 µM) in a concentration-dependent manner. The PGF 2α -induced increase in intracellular Ca 2+ concentration was also inhibited. The effects of capsaicin were readily reversed by washing capsaicin from the bath. Capsaicin-induced relaxation was not attenuated by pretreatment with capsazepine (1 µM), a blocker of vanilloid receptor or ruthenium red (1 µM), a blocker of non-selective cation channel. Previous exposure to a high concentration of capsaicin (100 µM) or repeated application of capsaicin did not eliminate the relaxation response to subsequent application of capsaicin. Increasing the external K + concentration to 80 mM significantly attenuated the capsaicin-induced relaxation with simultaneous change in intracellular Ca 2+ concentration. Pretreatment with iberiotoxin (100 nM), a blocker of Ca 2+ -activated K + channel, only partially inhibited the capsaicin-induced relaxation. However, application of 4-aminopyridine (4-AP, 1 mM), a blocker of delayed rectifier K + current significantly inhibited the capsaicin-induced relaxation with concomitant attenuation of the effect on intracellular Ca 2+ concentration. These results indicate that capsaicin may have a direct relaxing effect on the smooth muscle contractility, and relaxation may be due to activation of the 4-AP-sensitive, delayed rectifier K + channels in the rabbit coronary artery. Capsaicin is a pungent constituent of red peppers and known to activate sensory nerve fibers via vanilloid receptor [20,[22][23]. Acute administration of capsaicin releases neuropeptide from sensory nerve endings, such as substance P, calcitonin gene related peptide (CGRP) and neurokinin A [7,17]. These neuropeptides seem to play a role in the regulation of vascular and airway smooth muscle tone [8,16,21]. In addition to this neuropeptide-mediated effect, capsaicin has a diverse effect on smooth muscle contractility and ion channel activity, depending on the species and preparations. Capsaicin relaxes bronchial smooth muscle through activation of Ca 2+ -activated K + channels [5,24], or constricts cerebral arteries by increasing Ca 2+ influx [4]. The inhibitory effect of capsaicin on Ca 2+ and K + current in cultured aortic smooth muscle cells has also been reported [15].In coronary artery, capsaicin has been reported to increase coronary flow and decrease ischemic ventricular tachycardia which may be related to the decrease in Ca 2+ influx [2]. The authors suggest a protective role of capsaicin on the ischemic cardiac damage through maintaining adequate coronary flow. However, it is uncertain whether capsaicin relaxes coronary artery through the release of neuropeptide such as CGRP, or through the direct inhibition of coronary smooth muscle [3,8,15]. Thus, the primary...

ABSTRACT. Lysophosphatidylcholine (LPC), which exists abundantly in lipid fraction of oxidized low density lipoprotein, has been implicated in enhanced agonist-induced contraction and increase of intracellular Ca 2+ . The effect of LPC on the activity of delayed rectifier K + current (I dK ), which is a major determinant of membrane potential and vascular tone under resting condition, was examined in rabbit coronary smooth muscle cells using whole cell patch clamping technique. Application of LPC to the bath solution caused a concentration-dependent inhibition of I dK , and the concentration to produce half-maximal inhibition was 1.51 µM. This effect of LPC on I dK was readily reversed after washout of LPC in the bath. The steady-state voltage dependence of I dK was shifted to positive direction by both extra-and intracellular application of LPC. Staurosporine (100 nM) pretreatment significantly suppressed the LPC-induced inhibition of I dK . These results suggest that LPC inhibits I dK in rabbit coronary smooth muscle cells by a pathway that involves protein kinase C, and the LPC-induced inhibition of I dK may be, at least in part, responsible for the abnormal vascular reactivity in atherosclerotic coronary artery. KEY WORDS: coronary smooth muscle, delayed rectifier K + current, lysophosphatidylcholine, protein kinase C.

The mechanism of carbon monoxide (CO)-induced relaxation were investigated in the guinea-pig ileum. CO (10%) inhibited the 40 mM KCl-induced contraction. This effect was antagonized by ODQ (1 microM), a soluble guanylate cyclase inhibitor. In contrast, CO did not inhibit the 40 mM KCl-induced increase in cytosolic Ca2+ level ([Ca2+]i). Cumulative addition of KCl induced a graded increase in both [Ca2+]i and muscle tension. In the presence of CO, the increase in muscle tension was attenuated whereas the increase in [Ca2+]i was only slightly decreased. Thus, the [Ca2+]i-tension relationship constructed by cumulative addition of KCl shifted downwards in the presence of CO. Using the patch clamp, CO was found to have little effect on the peak Ba currents (I(Ba)) when voltage was stepped from -60 mV to 0 mV. From these results, we conclude that CO inhibits contraction of guinea-pig ileum mainly by the decrease in the sensitivity of contractile elements to Ca2+ via a cyclic GMP-dependent pathway but not by the inhibition of L-type Ca2+ channel.

It has been reported that a change in the cellular redox state may be involved in the regulation of vascular tone, but the underlying mechanism is not fully understood. The present study was designed to investigate the cellular effect of sulfhydryl modifying agents in the coronary artery of rabbit using the tension measurement and whole cell clamping method. The application of diamide, a sulfhydryl oxidizing agent, relaxed the endothelium denuded coronary arteries in a dose dependent manner. The fact that this diamide-induced relaxation was significantly attenuated by a pretreatment of 4-AP, and the coronary arteries precontracted with 100 mM K+ instead of histamine, suggests the involvement of 4-AP sensitive K+ channels in the diamide-induced relaxation of coronary arteries. Whole cell patch clamp studies revealed that the 4-AP sensitive IdK was significantly enhanced by the membrane permeant oxidizing agents, diamide and DTDP, and were reversed by subsequent exposure to the reducing agent, DTT. Neither the membrane impermeant oxidizing or reducing agents, GSSG or GSH, had any effect on the activity of IdK, indicating that intracellular sulfhydryl modification is critical for modulating IdK activity. The Diamide failed to significantly alter the voltage dependence of the activation and inactivation parameters, and did not change the inactivation process, suggesting that diamide increases the number of functional channels without altering their gating properties. Since IdK has been believed to play an important role in regulating membrane potential and arterial tone, our results about the effect of sulfhydryl modifying agents on coronary arterial tone and IdK activity should help understand the pathophysiology of the diseases, where oxidative damage has been implicated.