Depolarization-induced calcium influx in rat mesenteric small arterioles is mediated exclusively via mibefradil-sensitive calcium channels

1 In this study, intracellular Ca2+ was measured as the Fura-2 ratio (R) of fluorescence excited at 340 and 380 nm (F-340/F-380) in nonpressurized rat mesenteric small arterioles (circle divide (lumen diameter) 10-25 mum). The response to depolarization using 75 mM KCl was an increase in R from a baseline of 0.96+/-0.01 ([Ca2+](i) similar to74 nM) to 1.04+/-0.01 (similar to128 nM) (n = 80). 2 The response to 75 mM K+ was reversibly abolished in Ca2(+)-free physiological saline solution, whereas phentolamine ( 10 mM) or tetrodotoxin (1 muM) had no effects. LaCl3 (200 muM) inhibited 61+/-9% of the response. 3 A [K+]-response curve indicated that the Ca2+ response was activated between 15 and 25 mM K+. The data suggest that the Ca2+ response was caused by the activation of voltage-dependent Ca2+ channels. 4 Mibefradil use dependently inhibited the Ca2+ response to 75 mM K+ by 29+/-2% (100 nM), 73+/-7% (1 muM) or 89+/-7% (10 muM). Pimozide (500 nM) use dependently inhibited the Ca2+ response by 85+/-1%. 5 Nifedipine (1 muM) inhibited the Ca2+ response to 75 mM K+ by 41+/-12%. The response was not inhibited by calciseptine (500 nM), omega-agatoxin IVA (100 nM), omega-conotoxin MVIIA (500 nM), or SNX-482 (100 nM). 6 Using reverse transcriptase-polymerase chain reaction, it was shown that neither Ca(V)2.1a (P-type) nor Ca(V)2.1b (Q-type) voltage-dependent Ca2+ channels were expressed in mesenteric arterioles, whereas the Ca(V)3.1 (T-type) channel was expressed. Furthermore, no amplification products were detected when using specific primers for the beta(1b), beta(2), or beta(3) auxiliary subunits of high-voltage-activated Ca2+ channels. 7 The results suggest that the voltage-dependent Ca2+ channel activated by sustained depolarization in mesenteric arterioles does not classify as any of the high-voltage-activated channels (L-, P/Q-, N-, or R-type), but is likely to be a T-type channel. The possibility that the sustained Ca2+ influx observed was the result of a T-type window current is discussed. British Journal of Pharmacology (2004).