Mechanisms of cGMP-dependent mesangial-cell relaxation: a role for myosin light-chain phosphatase activation

Although the cGMP-dependent relaxation of contractile cells seems to depend on the ability of the cyclic nucleotide to interfere with intracellular ca lcium, this does not appear to be the only mechanism involved. The present experiments were designed to analyse alternative mechanisms, trying to test the hypothesis that cGMP could relax rat mesangial cells by activating myo sin light-chain phosphatase (MLC-PP), with the subsequent dephosphorylation of myosin light chain (MLC). The effect of a cGMP analogue, dibutyryl cGMP (dbcGMP), on angiotensin II-(AII) and PMA-induced MLC phosphorylation (MLC P) was tested, in the presence of calyculin A (CA), an inhibitor of MLC-PP. MLCP was measured, after cell labelling with P-32, by immunoprecipitation. dbcGMP prevented the increased MLCP induced by AII or PMA, and this inhibi tion was blocked by CA. dbcGMP also increased the MLC dephosphorylation obs erved in cells incubated with AII and in which MLC kinase and protein kinas e C activities were blocked. The AII-elicited increased intracellular calci um concentration was only partially inhibited by dbcGMP. These results sugg est that the cGMP-induced mesangial-cell relaxation could be due, at least partially, to the stimulation of MLC-PP.