Regulation of agonist-evoked Ca2+i oscillation by intracellular Ca2+ and Ba2+ in AR42J cells

Measurements of intracellular Ca2+ (Ca2+i) and intracellular Ba2+ (Ba2+i) in single AR42J cells were used to evaluate the effect of Ca2+i and Ba2+i on agonist-evoked Ca2+i oscillations. Variations in Ca2+i and Ba2+i were imposed by gradual activation of entry through voltage-activated Ca2+ channels (VACC) present in the plasma membrane of these cells. Activation of high K+ was followed by partial inactivation of the channels and stabilization of Ca2+i at a new steady-state level depending on the extent of depolarization. Activation by BAY K 8644 was followed by complete inactivation and return of Ca2+i to resting levels. Ba2+ activated the channels and entered the cells but could not be removed from the cytosol by cellular Ca2+ pumps. The use of channel blockers and the ability to increase Ca2+i and Ba2+i by channel activation during Ca2+i oscillations showed that VACC do not contribute to or are activated during agonist-stimulated Ca2+ oscillation in this cell type. Graded activation of VACC showed that an increase in Ca2+i between the spikes to below 200 nM increased the frequency of the oscillation. Further increase in Ca2+i caused gradual reduction in the frequency. At Ca2+i above 500 nM, Ca2+i oscillations were inhibited. The inhibitory but not the stimulatory effects of Ca2+i on the oscillations can be mimicked by Ba2+i. These observations suggest that Ca2+i levels between the spikes play an important role in regulating the oscillations.