Role of T-type calcium channels in myogenic tone of skeletal muscle resistance arteries

T-type calcium channels may be involved in the maintenance of myogenic tone. We tested their role in isolated rat cremaster arterioles obtained after CO2 anesthesia and decapitation. Total RNA was analyzed by RT-PCR and Southern blotting for calcium channel expression. We observed expression of voltage-operated calcium (Ca-v) channels Ca(v)3.1 (T-type), Ca(v)3.2 (T-type), and Ca(v)1.2 (L-type) in cremaster arterioles (n = 3 rats). Amplification products were observed only in the presence of reverse transcriptase and cDNA. Concentration-response curves of the relatively specific L-type blocker verapamil and the relatively specific T-type blockers mibefradil and nickel were made on cannulated vessels with either myogenic tone (75 mmHg) or a similar level of constriction induced by 30 mM K+ at 35 mmHg. Mibefradil and nickel were, respectively, 162-fold and 300-fold more potent in inhibiting myogenic tone compared with K+-induced constriction [log(IC50, M): mibefradil, basal -7.3 +/- 0.2 (n = 9) and K+ -5.1 +/- 0.1 (n = 5); nickel, basal - 4.1 +/- 0.2 (n = 5) and K+ - 1.6 +/- 0.5 (n = 5); means +/- SE]. Verapamil had a 17-fold more potent effect log(IC50, M): basal -6.6 +/- 0.1 (n = 5); K+ -5.4 +/- 0.3 (n = 4); all log(IC50) P < 0.05, basal vs. K+]. These data suggest that T-type calcium channels are expressed and involved in maintenance of myogenic tone in rat cremaster muscle arterioles.