Different mechanisms for [Ca2+]i oscillations induced by carbachol and high concentrations of [Ca2+]o in the rat ventricular myocyte

1. The purpose of the present study was to explore the different mechanisms of Ca2+]i oscillations induced by high concentrations of either carbachol (CCh) or extracellular Ca2+ (Ca2+]o). First, we compared the oscillations induced by CCh at concentrations of 100-300 micromol/L and Ca2+]o (5 mmol/L) in the single rat ventricular myocyte. Second, we studied CCh- and Ca2+]o-induced Ca2+]i oscillations following either interference with the production of inositol trisphosphate (IP3), reductions in cytosolic Ca2+ (Ca2+]i), inhibition of Ca2+ influx and Na+-Ca2+ exchange or depletion of Ca2+ from its intracellular store. 2. The Ca2+]i oscillations induced by CCh were frequent and were superimposed on Ca2+]i transients in electrically stimulated cells, whereas those induced by high Ca2+]o were occasional and occurred in quiescent cells and between Ca2+]i transients in electrically stimulated cells. In both cases, Ca2+]i oscillations were preceded by an increase in resting levels of Ca2+]i. 3. Carbachol-induced Ca2+]i oscillations were accompanied by an increase in amplitude and prolongation of the time of decline to 80% of the peak of the Ca2+]i transient, while high Ca2+]o-induced Ca2+]i oscillations were the opposite. 4. A reduction of Ca2+]o to 0.1 mmol/L and treatment with Ni2+ or ryanodine or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid AM (BAPTA-AM) abolished the Ca2+]i oscillations induced by both CCh and high Ca2+]o. 5. The calcium channel blockers verapamil and nifedipine and inhibitors of phospholipase C (neomycin and U-73122) abolished the Ca2+]i oscillations induced by CCh; Li+ accelerated the onset of the Ca2+]i oscillations induced by CCh. 6. These observations suggest that the mechanisms responsible for the Ca2+]i oscillations induced by CCh and high Ca2+]o are different from each other. Other than an increase in extracellular Ca2+ influx as a mechanism common for both CCh- and high Ca2+]o-induced Ca2+]i oscillations, the CCh-induced Ca2+]i oscillations involve influx of Ca2+ via L-type Ca2+ channels, Na+-Ca2+ exchange, mobilization of intracellular Ca2+ and IP3 production.