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...

Presynaptic imidazoline receptors (R i-pre ) are found in the sympathetic axon terminals of animal and human cardiovascular systems, and they regulate blood pressure by modulating the release of peripheral noradrenaline (NA). The cellular mechanism of R i-pre -induced inhibition of NA release is unknown. We, therefore, investigated the effect of R i-pre activation on voltage-dependent Ca 2+ channels in rat superior cervical ganglion (SCG) neurons, using the conventional whole-cell patch-clamp method. Cirazoline (30 μm), an R i-pre agonist as well as an α-adrenoceptor (R α ) agonist, decreased Ca 2+ currents (I Ca ) by about 50% in a voltage-dependent manner with prepulse facilitation. In the presence of low-dose rauwolscine (3 μm), which blocks the α 2 -adrenoceptor (R α2 ), cirazoline still inhibited I Ca by about 30%, but prepulse facilitation was significantly attenuated. This inhibitory action of cirazoline was almost completely prevented by high-dose rauwolscine (30 μm), which blocks R i-pre as well as R α2 . In addition, pretreatment with LY320135 (10 μm), another R i-pre antagonist, in combination with low-dose rauwolscine (3 μm), also blocked the R α2 -resistant effect of cirazoline. Addition of guanosine-5 -O-(2-thiodiphosphate) (2 mm) to the internal solutions significantly attenuated the action of cirazoline. However, pertussis toxin (500 ng ml −1 ) did not significantly influence the inhibitory effect of cirazoline. Moreover, cirazoline (30 μm) suppressed M current in SCG neurons cultured overnight. Finally, ω-conotoxin (ω-CgTx) GVIA (1 μm) obstructed cirazolineinduced current inhibition, and cirazoline (30 μm) significantly decreased the frequency of action potential firing in a partly reversible manner. This cirazoline-induced inhibition of action potential firing was almost completely occluded in the presence of ω-CgTx. Taken together, our results suggest that activation of R i-pre in SCG neurons reduced N-type I Ca in a pertussis toxinand voltage-insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.

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Transcutaneous electrical nerve stimulation (TENS) has widely been employed as a method of obtaining analgesia in medical practice. The mechanisms of pain relief by TENS are known to be associated with the spinal gate control mechanism or descending pain inhibitory system. However, most of the studies concerning the analgesic effects and their mechanisms for TENS have dealt with somatic pain. Thus, in this experiment, we investigated the analgesic effects of TENS on renal pain as a model of visceral pain, and the characteristics of the dorsal horn cells with renal inputs. The renal pain was induced by acute occlusion of the ureter or renal artery. The main results are summarized as follows: 1) The renal nerve was composed of A beta, A delta and C fiber groups; the thresholds for each group were 400-800 mV, 1.1-1.5 V, and 2.1-5.8 V, respectively. 2) The dorsal horn cells tested received A and/or C afferent fibers from the kidney, and the more C inputs the dorsal horn cells had, the greater was the response to the stimuli that elicited the renal pain. 3) 94.9% of cells with renal input had the concomitant somatic receptive fields on the skin; the high threshold (HT) and wide dynamic range (WDR) cells exhibited a greater responses than low threshold (LT) cells to the renal pain-producing stimuli. 4) TENS reduced the C-responses of dorsal horn cells to 38.9 +/- 8.4% of the control value and the effect lasted for 10 min after the cessation of TENS. 5) By TENS, the responses evoked by acute occlusion of the ureter or renal artery were reduced to 37.5 +/- 9.7% and 46.3 +/- 8.9% of the control value, respectively. This analgesic effects lasted 10 min after TENS. 6) The responses elicited by squeezing the receptive fields of the skin were reduced to 40.7 +/- 7.9% of the control value and the effects lasted 15 min after TENS. These results suggest that most of dorsal horn cells with renal inputs have the concomitant somatic inputs and TENS can alleviate the renal pain as well as somatic pain.

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.