Abstract
The effects of caffeine on agonist-induced changes in intracellular Ca2+ concentration ([Ca2+]i) were studied in single fura 2-loaded cells and suspensions of rat hepatocytes. In single cells, caffeine (5-10 mM) inhibited [Ca2+]i oscillations induced both by noradrenaline (0.1 microM) and by vasopressin (0.1 nM). Caffeine shifted the dose-response curves of the [Ca2+]i rise induced by vasopressin (0.5 to 2 nM) and noradrenaline (from 80 to 580 nM) in suspensions of liver cells loaded with quin2. This inhibitory effect of caffeine was not due to inhibition of phosphodiesterase enzymes and elevation of cyclic AMP levels, because application of 3-isobutyl-1-methylxanthine, forskolin or 8-bromo cyclic AMP had no inhibitory effect on the intracellular Ca2+ rise induced by inositol 1,4,5-trisphosphate (InsP3)-dependent agonists. We demonstrate that the inhibitory effect of caffeine may result from at least three actions of caffeine: (1) inhibition of receptor-stimulated InsP3 formation; (2) inhibition of agonist-stimulated Ca2+ influx; and (3) direct inhibition of the InsP3-sensitive Ca(2+)-release channel.