Effects of mibefradil and nifedipine on arteriolar myogenic responsiveness and intracellular Ca²⁺

Abstract

Ca2+ entry mechanisms underlying spontaneous arteriolar tone and acute myogenic reactivity remain uncertain. These studies aimed to compare the effects of nifedipine and the putative T‐channel blocker, mibefradil, on arteriolar myogenic responsiveness and intracellular Ca2+ (Ca2+i). First order cremaster muscle arterioles (1A) were isolated from rats, cannulated, pressurized to 70 mmHg in the absence of intraluminal flow, and mechanical responses studied by video microscopy. The Ca2+i was measured using fluorescence imaging of Fura 2 loaded arterioles. Both nifedipine and mibefradil showed dose‐dependent inhibition of spontaneous myogenic tone (at 70 mmHg; pEC50 7.04±0.17 vs 6.65±0.20 respectively, n=6 for both, n.s.) and KCl‐induced vasoconstriction (at 70 mmHg; pEC50 6.93±0.38 vs 6.45±0.27 respectively, n=6 for both, n.s.). In arterioles maintained at 50 mmHg, nifedipine (10−7 and 10−5 M) caused a concentration dependent reduction in Ca2+i, however, mibefradil (10−7 and 10−5 M) had no effect. Furthermore nifedipine significantly attenuated the increase in Ca2+i associated with an acute pressure step (50–120 mmHg) whereas mibefradil was considerably less effective. Mibefradil (10−7 M) significantly attenuated contractile responses to 60 mM KCl without altering the KCl‐induced increase in Ca2+i, in contrast to nifedipine (10−7 M) which reduced both Ca2+i and contraction. Membrane potential of arterioles with spontaneous myogenic tone (70 mmHg) was −41.5±1.0 mV. Nifedipine (10−7 or 10−5 M) had no effect on membrane potential, however mibefradil (10−5 M) caused significant depolarization. In summary, both mibefradil and nifedipine inhibit arteriolar spontaneous tone and acute myogenic reactivity. While there may be overlap in the mechanisms by which these agents inhibit tone, differences in effects on membrane potential and intracellular Ca2+ levels suggest mibefradil exhibits actions other than blockade of Ca2+ entry in skeletal muscle arterioles. British Journal of Pharmacology (2000) 131, 1065–1072; doi:

Antihypertensive Effects of the Putative T-Type Calcium Channel Antagonist Mibefradil Are Mediated by the L-Type Calcium Channel Ca v 1.2

“…This notion is in excellent agreement with the recent finding that deletion of the Ca v 3.2 T-type channels impaired coronary relaxation, not contraction, in mice. 15 Given the controversial literature on the role of T-type Ca 2ϩ channel as a target for cardiovascular pathology, 6,14,15,[22][23][24][25] studies on transgenic mice lacking T-type Ca 2ϩ channels without the need to rely on the unspecific blocker mibefradil might help to further clarify the functional role of these channels. …”

“…Poly(A) mRNA was isolated from the vessels using Dynabeads Oligo(dT) 25 The mRNA was eluted with diethylpyrocarbonate-treated water. Oligo-dT primers and Superscript Reverse Transcriptase II (Life Technologies) were used for cDNA synthesis.…”

Reduced EDHF responses and connexin activity in mesenteric arteries from the insulin-resistant obese Zucker rat

“…The chamber was superfused with Krebs solution at 37°C, at a rate of 4 ml/min, and the vessel was pressurised to 80 mmHg over 60 min. Vessel length was adjusted as previously described [30]. Pressure and intraluminal diameter measurements were recorded on a MacLab system (MacLab/4 Chart Software, model MKIIII; AD Instruments, Sydney, NSW, Australia).…”

Mechanism of Contraction of Rat Isolated Tail Arteries by Hyposmotic Solutions

“…Ca v 1.2 is an important route of Ca 2+ entry into vascular smooth muscle [41]and myogenic tone has been reported to be abolished by Ca v 1.2 antagonists, such as nifedipine, verapamil and nicardipine in a variety of different arteries [11, 37, 42, 43]. In further support of a role for Ca v 1.2 in the myogenic response, Bay K-8644, an activator of Ca v 1.2 increased myogenic responses in rat mesenteric arteries [37]and in mice with an inactivated Ca v 1.2 gene, myogenic responses failed to develop [44].…”