PERMISSIVE ROLE OF CAMP IN THE OSCILLATORY CA2-TRISPHOSPHATE IN RAT HEPATOCYTES( RESPONSE TO INOSITOL 1,4,5)

Rat hepatocytes respond to alpha-adrenergic stimulation by intracellul ar production of myo-inositol 1,4,5-trisphosphate (IP3) which stimulat es the periodic release and reuptake of intracellular store (IS) Ca2+. The generation of these Ca2+ oscillations was investigated by simulta neously monitoring Ca2+ changes in the cytosol and IS by combined fluo rescence microscopy and whole-cell patch clamp. Intracellular IP3 perf usion (1-50 mu M in the pipette) produced three types of Ca2+ response : understimulation, oscillations and overstimulation, i.e. with Ca2levels not returning to baseline. In a total of 57 experiments, only t hree displayed oscillations during continuous IP3 infusion, in a narro w range of IP3 concentration centred around 5-8 mu M in the pipette. I n oscillating cells, cytosolic Ca2+ spikes were synchronized with tran sient Ca2+ depletions of the IS, consistent with a direct exchange of Ca2+ between the two compartments. Application of 8-Br-cAMP to cells i nfused with IP3 increased the probability of eliciting Ca2+ oscillatio ns by a factor of 4-5 for IP3 concentrations in the range 1-10 mu M, w hereas IP3 concentrations above 10 mu M always resulted in overstimula tion. IP3 photorelease experiments and measurements of IS Ca2+ content indicated that 8-Br-cAMP enhanced the affinity of the IP3 receptor an d increased the pool of releasable Ca2+. We propose that cAMP has a pe rmissive role in the generation of IP3-induced Ca2+ oscillations by ex tending the window of IP3 concentrations able to elicit oscillations.