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

In this study, intracellular Ca 2+ 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 (Ø (lumen diameter) 10–25 μ m). The response to depolarization using 75 m M KCl was an increase in R from a baseline of 0.96±0.01 ([Ca 2+ ] i ∼74 n M ) to 1.04±0.01 (∼128 n M ) ( n =80). The response to 75 m M K + was reversibly abolished in Ca 2+ ‐free physiological saline solution, whereas phentolamine (10 μ M ) or tetrodotoxin (1 μ M ) had no effects. LaCl 3 (200 μ M ) inhibited 61±9% of the response. A [K + ]‐response curve indicated that the Ca 2+ response was activated between 15 and 25 m M K + . The data suggest that the Ca 2+ response was caused by the activation of voltage‐dependent Ca 2+ channels. Mibefradil use dependently inhibited the Ca 2+ response to 75 m M K + by 29±2% (100 n M ), 73±7% (1 μ M ) or 89±7% (10 μ M ). Pimozide (500 n M ) use dependently inhibited the Ca 2+ response by 85±1%. Nifedipine (1 μ M ) inhibited the Ca 2+ response to 75 m M K + by 41±12%. The response was not inhibited by calciseptine (500 n M ), ω ‐agatoxin IVA (100 n M ), ω ‐conotoxin MVIIA (500 n M ), or SNX‐482 (100 n M ). 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 Ca 2+ 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 β 1b , β 2 , or β 3 auxiliary subunits of high‐voltage‐activated Ca 2+ channels. The results suggest that the voltage‐dependent Ca 2+ 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 Ca 2+ influx observed was the result of a T‐type window current is discussed.British Journal of Pharmacology (2004) 142 , 709–718. doi: 10.1038/sj.bjp.0705841