Voltage dependence of Ca2+ sparks in intact cerebral arteries

Ca2+ sparks have been previously described in isolated smooth muscle cells. Here we present the first measurements of local Ca2+ transients ("Ca2+ sparks") in an intact smooth muscle preparation. Ca2+ sparks appear to result from the opening of ryanodine-sensitive Ca2+ release (RyR) channels in the sarcoplasmic reticulum (SR). Intracellular Ca2+ concentration ([Ca2+]i) was measured in intact cerebral arteries (40-150 micron in diameter) from rats, using the fluorescent Ca2+ indicator fluo 3 and a laser scanning confocal microscope. Membrane potential depolarization by elevation of external K+ from 6 to 30 mM increased Ca2+ spark frequency (4. 3-fold) and amplitude (approximately 2-fold) as well as global arterial wall [Ca2+]i (approximately 1.7-fold). The half time of decay ( approximately 50 ms) was not affected by membrane potential depolarization. Ryanodine (10 microM), which inhibits RyR channels and Ca2+ sparks in isolated cells, and thapsigargin (100 nM), which indirectly inhibits RyR channels by blocking the SR Ca2+-ATPase, completely inhibited Ca2+ sparks in intact cerebral arteries. Diltiazem, an inhibitor of voltage-dependent Ca2+ channels, lowered global [Ca2+]i and Ca2+ spark frequency and amplitude in intact cerebral arteries in a concentration-dependent manner. The frequency of Ca2+ sparks (<1 s-1 . cell-1), even under conditions of steady depolarization, was too low to contribute significant amounts of Ca2+ to global Ca2+ in intact arteries. These results provide direct evidence that Ca2+ sparks exist in quiescent smooth muscle cells in intact arteries and that changes of membrane potential that would simulate physiological changes modulate both Ca2+ spark frequency and amplitude in arterial smooth muscle.