Phorbol ester-stimulated bidirectional transmembrane calcium flux in A7r5 vascular smooth muscle cells

In A7r5 vascular smooth muscle cells, the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) caused a marked increase in the rate of unidirectional Ca2+ influx and a 63 +/- 9% increase in net 30-min 45Ca2+ uptake. Maximal TPA-stimulated 45Ca2+ uptake occurred at concentrations less than or equal to 3 nM and was equivalent to the maximal uptake stimulated by 55 mM KCl or 1 microM Bay k 8644. TPA-stimulated Ca2+ uptake was not additive to KCl- or Bay k 8644-stimulated uptake, and was inhibited by verapamil (100 microM), nitrendipine (1 microM), or high concentrations of Bay k 8644 (greater than or equal to 10 microM). The biologically inactive phorbol ester 12-O-tetradecanoyl phorbol-13-acetate methyl ether (10 nM) had no effect on 45Ca2+ uptake. TPA caused a 43 +/- 12% increase in Ca2+ efflux in 30 min, and exposure to TPA (10 nM) for 5 and 30 min caused no significant change in net cellular Ca2+ content as determined by radioisotopic equilibration or atomic absorption spectroscopy. TPA caused no apparent change in intracellular free Ca2+ concentration as determined with quin 2. Thus, in A7r5 cells, TPA causes a marked increase in Ca2+ influx through channels with the pharmacological characteristics of dihydropyridine-sensitive, voltage-dependent channels. This TPA-stimulated Ca2+ uptake is associated with increased Ca2+ efflux and no significant change in net cellular Ca2+ content or intracellular free Ca2+ concentration.