Abstract
Cremaster muscle arteriolar smooth muscle cells (SMCs) display inositol 1,4,5-trisphosphate receptor-dependent Ca(2+) waves that contribute to global myoplasmic Ca(2+) concentration and myogenic tone. However, the contribution made by voltage-gated Ca(2+) channels (VGCCs) to arteriolar SMC Ca(2+) waves is unknown. We tested the hypothesis that VGCC activity modulates SMC Ca(2+) waves in pressurized (80 cmH(2)O/59 mmHg, 34°C) hamster cremaster muscle arterioles loaded with Fluo-4 and imaged by confocal microscopy. Removal of extracellular Ca(2+) dilated arterioles (32 ± 3 to 45 ± 3 μm, n = 15, P < 0.05) and inhibited the occurrence, amplitude, and frequency of Ca(2+) waves ( n = 15, P < 0.05), indicating dependence of Ca(2+) waves on Ca(2+) influx. Blockade of VGCCs with nifedipine (1 μM) or diltiazem (10 μM) or deactivation of VGCCs by hyperpolarization of smooth muscle with the K(+) channel agonist cromakalim (10 μM) produced similar inhibition of Ca(2+) waves ( P < 0.05). Conversely, depolarization of SMCs with the K(+) channel blocker tetraethylammonium (1 mM) constricted arterioles from 26 ± 3 to 14 ± 2 μm ( n = 11, P < 0.05) and increased wave occurrence (9 ± 3 to 16 ± 3 waves/SMC), amplitude (1.6 ± 0.07 to 1.9 ± 0.1), and frequency (0.5 ± 0.1 to 0.9 ± 0.2 Hz, n = 10, P < 0.05), effects that were blocked by nifedipine (1 μM, P < 0.05). Similarly, the VGCC agonist Bay K8644 (5 nM) constricted arterioles from 14 ± 1 to 8 ± 1 μm and increased wave occurrence (3 ± 1 to 10 ± 1 waves/SMC) and frequency (0.2 ± 0.1 to 0.6 ± 0.1 Hz, n = 6, P < 0.05), effects that were unaltered by ryanodine (50 μM, n = 6, P > 0.05). These data support the hypothesis that Ca(2+) waves in arteriolar SMCs depend, in part, on the activity of VGCCs. NEW & NOTEWORTHY Arterioles that control blood flow to and within skeletal muscle depend on Ca(2+) influx through voltage-gated Ca(2+) channels and release of Ca(2+) from internal stores through inositol 1,4,5-trisphosphate receptors in the form of Ca(2+) waves to maintain pressure-induced smooth muscle tone.