Abstract
To determine whether insulin-like growth factor II (IGF-II) activates phospholipase C in the basolateral membrane of the renal proximal tubular cell, we incubated basolateral membranes isolated from canine kidney with rat IGF-II (rIGF-II) and measured levels of inositol trisphosphate (Ins-P3) in suspensions and of diacylglycerol extractable from the membranes. Incubation with rIGF-II increased levels of Ins-P3 and diacylglycerol in a concentration-dependent manner. Significant enhancement of Ins-P3 levels and extractable diacylglycerol occurred in suspensions incubated with as little as 10(-10) M rIGF-II. Elevated levels of Ins-P3 were measured after as little as 5 sec of incubation. Increases were no longer detectable after 45 sec of incubation, due to dephosphorylation of Ins-P3 in membrane suspensions. Incubation with either insulin or insulin-like growth factor I did not affect the level of Ins-P3. IGF-II-stimulated increases in Ins-P3 did not occur when basolateral membranes were suspended in the absence of free calcium. Increases were demonstrable in basolateral membrane suspensions in 0.1, 0.2, or 0.3 microM calcium, but not in 1.0 microM calcium. Inclusion of guanosine 5'-[gamma-thio]triphosphate in incubation mixtures did not increase levels of Ins-P3, nor did it enhance the action of rIGF-II in this regard. However, inclusion of guanosine 5'-[beta-thio]diphosphate inhibited rIGF-II stimulation of Ins-P3 production. In contrast to findings with basolateral membrane suspensions, incubation with rIGF-II did not increase levels of Ins-P3 in suspensions of isolated brush-border membranes. Our data are consistent with IGF-II-mediated activation of phospholipase C in isolated proximal tubular basolateral membranes. Such an action could reflect the mechanism by which the IGF-II "signal" is transmitted across the basolateral membrane of the renal proximal tubular cell and by which the actions of this peptide are mediated in renal and non-renal cells.