STIMULATED CA2- CURRENTS AFTER RELEASING CA2+ FROM THE INTRACELLULAR STORE IN SUBMANDIBULAR-GLAND CELLS OF THE RAT( ENTRY ACTIVATES CL)

In order to examine whether Ca2+ entry is directly involved in control ling exocrine secretion, the Ca2+-activated Cl- currents were recorded in single and clusters of rat submandibular gland cells using the who le-cell patch-clamp method. Extracellularly applied acetylcholine (ACh , 10 nM) as well as intracellularly applied GTP gamma S and InsP(3) ca used repetitive transients of the Cl- currents activated by intracellu lar Ca2+ These responses occurred also in the absence of external Ca2, but disappeared after several minutes. Readmission of Ca2+ to the ex tracellular solution restored the repetitive current transients, while introduction of Sr2+ failed to restore the current signals in spite o f the presence of Sr2+ entry detected by microfluorimetry. On the othe r hand, direct application of Sr2+ to the cell inside caused activatio n of the Cl- currents although less effectively than Ca2+. When Ca2+ w as introduced to the extracellular solution during an interruption of ACh stimulation after the ACh-induced depletion of intracellular Ca2store, the Cl- current was not elicited. However, a subsequent challen ge with ACh at the same concentration in the absence of extracellular Ca2+ caused repetitive transient Cl- currents. The results suggest tha t in this cell type the stimulated Ca2+ entry does not by itself activ ate the Cl- currents but activates them indirectly by triggering Ca2release from the intracellular Ca2+ store which may take up Ca2+ soon after the Ca2+ entry.