SYNCHRONIZED CALCIUM SPIKING RESULTING FROM SPONTANEOUS CALCIUM ACTION-POTENTIALS IN MONOLAYERS OF NRK FIBROBLASTS

The correlation between the intracellular Ca2+ concentration ([Ca2+](i )) and membrane potential in monolayers of density-arrested normal rat kidney (NRK) fibroblasts was investigated. Using the fluorescent prob e Fura-2, spontaneous repetitive spike-like increases in [Ca2+](i) (Ca 2+ spikes) were observed that were synchronised throughout the entire monolayer. Ca2+ spikes disappeared in Ca2+-free solutions and could be blocked by the L-type Ca2+ channel antagonist felodipine. Simultaneou s measurements of [Ca2+](i) and membrane potential showed that these C a2+ spikes were paralleled by depolarisations of the plasma membrane. Using patch clamp measurements, action potential-like depolarisations consisting of a fast spike depolarisation followed by a plateau phase were seen with similar kinetics as the Ca2+ spikes. The action potenti als could be blocked by L-type Ca2+ channel blockers and were dependen t on extracellular Ca2+. The plateau phase was predominantly determine d by a Cl- conductance and was dependent on intracellular Ca2+. The pr esence of voltage-dependent L-type Ca2+ channels in NRK cells was conf irmed by patch clamp measurements in single cells. It is concluded tha t monolayers of density-arrested NRK fibroblasts exhibit spontaneous C a2+ action potentials leading to synchronised Ca2+ spiking. This excit ability of monolayers of fibroblasts may represent a novel Ca2+ signal ing pathway in electrically coupled fibroblasts, cells that were hithe rto considered to be inexcitable.