Smooth Muscle

Abstract

The underlying mechanism of pathogenesis of hypertension has been intensely investigated but remains largely unknown. Hypertension is generally thought to be associated with the disorder of Ca 2 + regulation in vascular smooth muscle such as the increase of Ca 2 + influx, the decrease of Ca 2 + efflux , or the change of sensitivity of contractile protein to Ca2+, etc. An increase of Ca2+ influx may be due to an increased Ca 2 + channel activity but the causal relationships of Ca2+ channel activity and hypertension in various hypertension models including diabetes mellitus-induced hypertension (DMHT) is not clearly established. For stable induction of DMHT, the left kidney was removed from a 6-week -old rat under pentobarbitone anaesthesia (30 mg kg-I). Two weeks later, 60 mg kg -1 of streptozotocin was injected into the peritoneal cavity to induce DM. After 1 week, urine was examined using Diastick® (Tokyo, Japan) to test whether DM was induced. Usually after 2 weeks, HT was fully induced. Nephrectomy or the induction of DM alone usually did not produce a comparable increase in blood pressure. For isolating vascular smooth muscle cells (VSMCs), we used mesenteric arterioles (3rd to 5th branch of mesenteric artery) with a diameter less than 150 p..m. VSMCs were isolated enzymatically. Ca 2 + current was measured using the whole-cell patch-clamp technique. All experiments were performed at 37°C. The membrane capacitance of VSMCs of DMHT is larger than that of control VSMCs (36·6 ± 3·64 pF, n = 31 vs. 22·4 ± 1·29 pF, n = 28, means ± S.E.M., P < 0·01, Student's t test). The Ca 2 + current amplitude in VSMCs of DM hypertensive rat is much larger than that in VSMCs of normotensive age-matched rats (575 ± 45 pA, n = 13 vs. 209 ± 20 pA, n = 19, P < 0·01). After Ca 2 + current amplitude was normalized by membrane capacitance, the normalized current amplitude in DMHT is 16·44 ± 1·62 pA pF-\ n = 13 (9·6 ± 1'18 pA pF-1 in normotensive rat, n = 19, P< 0·01), which is a 167 % increase in current density. There were no noticeable diffe~ences in the steady-state activation and inactivation of Ca 2 + current between normotensive and DMHT VSMCs . From these results, the DMHT is associated with an increase of Ca 2 + current density. However, further study is required to establish why the Ca2+ current is so large, and whether an increased Ca 2 + influx through the Ca 2 + channel contributes to the increase of intracellular Ca 2 +, leading to the development of hypertension in VSMC of DMHT. (1999) A large transient outward current (STOC) following an inward Ca 2 + current was recorded in Tyrode solution when cells were clamped to -30 m V onwards from a holding potential of -60 mv' The STOC displayed an N-shaped I-V relationship. It was absent when the extracellular Ca 2 + was removed or replaced by Ba 2 +, and inhibited by 3 mM extracellular TEA or intracellular Cs +. Verapamil (20 p..M), a Ca 2 + channel blocker, suppressed the current by 78-90 % at voltages between -20 and +50 mV (P < 0'05, n = 11). The BKea specific blocker ibe...