Agonist concentration influences the pattern and time course of intracellular Ca2+ oscillations in human arterial smooth muscle cells

Oscillations in intracellular Ca²⁺ were recorded in cultured human uterine artery vascular smooth muscle cells. In the absence of external Ca²⁺, prolonged application of 3 M histamine activated a large transient increase in Ca²⁺ followed by a burst of Ca²⁺ spikes. The time course and frequency of the spikes were approximately constant until the last two to three spikes, when the inter-spike interval progressively increased. At 30 M histamine the response was different; the amplitude of the spikes decreased rapidly to zero, the rate of rise of successive transients fell and the time between spikes increased. The cessation of oscillatory activity was not associated with the depletion of intracellular Ca²⁺ stores, since increased doses of agonist or the sulphydryl reagent thimerosal could reactivate Ca²⁺ release. The changes in the pattern of intracellular Ca²⁺ spikes seen with increasing agonist concentration may reflect the involvement of different inactivation mechanisms in the termination of Ca²⁺ transients. In the presence of external Ca²⁺, histamine (3–30 M) activated regular Ca²⁺ oscillations. The frequency, but not the amplitude, of the oscillations was dependent on agonist concentration, the highest frequency of spiking was observed at 30 M histamine. In cells depolarised with 30 mM K⁺, histamine was still able to activate Ca²⁺ oscillations, but the dependence of spike frequency upon agonist concentration was abolished. Ca²⁺ oscillations could be activated in the presence of verapamil and nifedipine (10 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.