Agonists which stimulate the inositol 1,4,5 trisphosphate ([1,4,5]-IP,)-dependent mobilization of Ca2+ from intracellular stores also stimulate entry of divalent cations across the cell membrane. Under appropriate experimental conditions, divalent cation entry across the cell membrane can be monitored as the rate at which the intracellular fluorescence of divalent cation indicators is quenched by the addition of Mn2+ to the extracellular medium. We report that addition of vasopressin to fura-2-loaded glomerular mesangial cells in culture markedly accelerated the rate at which Mn2+ quenched fura-2 fluorescence at its Ca2+insensitive wavelength in the presence of extracellular NaCI, but that this quench response was attenuated when CI-was removed from the extracellular medium by equimolar substitution with impermeant anions (gluconate, methanesulfonate, acetate, lactate). Similarly, loss of agonist-induced quench also occurred when CI-was substituted with gluconate in K+-containing media. Addition of the CIchannel inhibitor, 5-nitro-2-(3-phenylpropylaminobenzoic acid) (NPPB), also inhibited Mn2 +-induced quench of fura-2 fluorescence following vasopressin addition. in contrast, in the presence of gramicidin to provide an alternate conductance pathway to accompany divalent cation entry, agonist-dependent Mn2+ quench occurred even in the absence of extracellular CI-, indicating that the requirement for CI-was not the result of cotransport on a common transporter nor the result of CI-serving as a necessary cofactor for divalent cation entry. A similar dependence on extracellular CI-was observed for other Ca'+-mobilizing agonists such as endothelin, as well as the intracellular Ca2+ ATPase inhibitor, thapsigargin. Extracellular CIdependence for agonist-induced divalent cation entry was also reflected in a corresponding extracellular CI-dependence for agonist-induced mesangial cell contraction. it has been previously shown by ourselves (Kremer et al., 1992a, Am. J. Physiol., 262:F668-F678) and others that agonist-stimulated calcium mobilization in mesangial cells i s accompanied by inhibition of K+ conductance and increased CIconductance. Accordingly, we conclude that the current findings suggest that activation of CI-conductance provides regulated charge compensation for receptor-mediated divalent cation entry in response to Ca2+-mobilizing vasoconstrictor agonists in mesangial cells.