AGONIST CONCENTRATION INFLUENCES THE PATTERN AND TIME-COURSE OF INTRACELLULAR CA2-MUSCLE CELLS( OSCILLATIONS IN HUMAN ARTERIAL SMOOTH)

Oscillations in intracellular Ca2+ were recorded in cultured human ute rine artery vascular smooth muscle cells. In the absence of external C a2+, prolonged application of 3 mu M histamine activated a large trans ient increase in Ca2+ followed by a burst of Ca2+ spikes. The time cou rse and frequency of the spikes were approximately constant until the last two to three spikes, when the inter-spike interval progressively increased. At 30 mu M histamine the response was different; the amplit ude of the spikes decreased rapidly to zero, the rate of rise of succe ssive transients fell and the time between spikes increased. The cessa tion of oscillatory activity was not associated with the depletion of intracellular Ca2+ stores, since increased doses of agonist or the sul phydryl reagent thimerosal could reactivate Ca2+ release. The changes in the pattern of intracellular Ca2+ spikes seen with increasing agoni st concentration may reflect the involvement of different inactivation mechanisms in the termination of Ca2+ transients. In the presence of external Ca2+, histamine (3-30 mu M) activated regular Ca2+ oscillatio ns. The frequency, but not the amplitude, of the oscillations was depe ndent on agonist concentration, the highest frequency of spiking was o bserved at 30 mu M histamine. In cells depolarised with 30 mM K+, hist amine was still able to activate Ca2+ oscillations, but the dependence of spike frequency upon agonist concentration was abolished. Ca2+ osc illations could be activated in the presence of verapamil and nifedipi ne (10 mu M). These data suggest that in human uterine artery vascular smooth muscle cells histamine-induced Ca2+ oscillations are generated largely by a ''cytosolic oscillator'' and are modified by the influx of Ca2+ across the surface membrane.