Biochemical signalling responses of cultured neonatal rat heart myocytes to angiotensin II

Abstract

Angiotensin II (AngII) has previously been shown to increase the beating frequency, decrease the force of contraction and induce phosphoinositide hydrolysis in cultured neonatal heart myocytes. These responses to AngII were transient, after which the cells became insensitive to any further application of the hormone. When the myocytes were incubated with 100 nM AngII there was a marked increase in total inositol phosphate (IPs) levels, that was maximal after 1 min, and returned to basal values after 5 min. This rapid desensitization process was concentration independent, and was completely reversible after 1 hr of hormone removal. The phenomenon was not due to an increase in the rate of degradation of inositol phosphates or depletion of the phospholipid pools. The homologous nature of the AngII-evoked desensitization, provided evidence that the receptor was the target of modification in this process. Binding studies with ({dollar}\sp{lcub}125{rcub}{dollar}I) AngII, revealed that the loss of surface receptors was not responsible for desensitization. Although activation of protein kinase C (PKC), desensitized the cells to AngII a role for the kinase in the hormone-evoked desensitization is not likely since the phenomenon was still observed in PKC-down regulated. The initial rapid burst of AngII evoked phosphoinositide turnover, was followed by a sustained (3 hr) low level of turnover. It was hypothesized that this long term signal may mediate a growth effect of the hormone on the heart cells. Consistent with this view, it was found that AngII increased the level of c-fos mRNA maximally after 0.5 hr. This response to the hormone was dose-dependent and mediated by the subtype-1 receptor through both PKC-dependent and independent pathways, but not through the initial 30s burst of inositol phosphates accumulation. The PKC-independent pathway was also independent of extracellular Ca{dollar}\sp{lcub}2+{rcub}{dollar}, arachidonic acid metabolites, calmodulin and the N{dollar}\sp{lcub}+{rcub}{dollar}/H{dollar}\sp{lcub}+{rcub}{dollar} exchanger. In conclusion, AngII induces a short lived, rapid hydrolysis of phosphoinositides, followed by a slower sustained effect in neonatal rat cardiac myocytes. The mechanism of desensitization of the cells to the former effect of the hormone is at the level of the receptor, but is independent of loss of surface receptors of PKC activation. (Abstract shortened with permission of author.)