MODULATION OF VOLTAGE-DEPENDENT CA2- CONCENTRATION IN RAT LACTOTROPHS( CONDUCTANCE BY CHANGING CL)

In pituitary cells, voltage-dependent Ca2+ channels play an important role in such physiological processes as exocytosis, secretion, the cel l cycle, and proliferation. Thus mechanisms that modulate voltage-depe ndent Ca2+ channel activity participate indirectly in regulating intra cellular Ca2+ concentration. We have shown a new modulating mechanism for voltage-dependent Ca2+ channels by demonstrating that Ca2+ influx is influenced by Cl-. To evaluate the role of Cl- on Ca2+ conductance coupling, we first measured the intracellular Cl- concentration of rat lactotrophs using the Cl--sensitive fluorescence probe sulfopropylqui nolinium by simple microspectrofluorometry or combined with electrophy siology. We found an average intracellular Cl- concentration of rat la ctotrophs of similar to 60 mM (n = 39). Using the whole cell tight-sea l recording technique, ave showed that a reduction in external Cl- con centration ([Cl-](o)) and a decrease in Cl- conductances affected Ca2 conductance as measured by Ba2+ movement through the Ca2+ channels (I -Ba). Low [Cl-](o) (39 mM) induced a decrease in Ca2+ entry via voltag e-gated Ca2+ channels (-27.75 +/- 4% of normalized I-Ba). Similarly, b lockade of the Cl- conductance by 1 mM 9-anthracene carboxylic acid in duced a decrease in I-Ba (-26 +/- 6% of normalized I-Ba). This modulat ion of I-Ba was inhibited by 24-h pretreatment of the cells with pertu ssis toxin (1 mu g/ml), suggesting that changes in Cl- concentration i nduced by low [Cl-], and 9-anthracene carboxylic acid interfered with the phosphorylation of G proteins involved in Ca2+ channel activation. These results suggest a feedback mechanism based on constant interact ion between Ca2+ and Cl-. Finally, they also emphasize the physiologic al role of Cl- In rat lactotrophs.