Calcium currents in the A7r5 smooth muscle-derived cell line.

We have studied voltage-dependent calcium channels in the A7r5 smooth muscle cell line by measuring the high-affinity binding of radiolabelled dihydropyridines (DHPs), whole-cell and single-channel currents in patch-clamped cells, as well as cytosolic calcium ([Ca2+]i) in fura-2-loaded cell suspensions and monolayers. Intact A7r5 cells express saturable, high-affinity, voltage-sensitive DHP binding sites with pharmacological properties characteristic of L-type calcium channels. When cells were voltage clamped in the whole-cell configuration with near normal intra- and extracellular solutions, a DHP-sensitive inward current resembling the L-type calcium current was dominant. With barium (10 mM) as the charge carrier, peak inward currents were typically recorded at test potentials between 0 and +20 mV. Currents were blocked by extracellular cadmium with a half-maximal inhibitory concentration of approximately 1 microM. Isoproterenol (1 microM) or forskolin (10 microM) increased currents in approximately half of the cells tested. Forskolin (10 microM) increased single-channel activity in five of eight cell-attached patches. After cells had been quiescent for several weeks, cell suspensions showed changes in resting [Ca2+]i in response to DHPs and increased potassium. Most confluent monolayers of cells showed spontaneous transient elevations in [Ca2+]i. Bath application of Bay K 8644 increased the frequency and magnitude of these [Ca2+]i transients, whereas nifedipine abolished the transients. These data suggest that the [Ca2+]i transients were due to synchronous action potentials in electrically coupled cell monolayers.