In adult guinea pig hearts, mechanical stretch activates PI hydrolysis via the local renin-angiotensin system, while PKC epsilon translocation occurs through both Ang II-dependent and independent mechanisms.
Stretch of neonatal cardiomyocytes activates phospholipase C with production of inositol trisphosphate and diacylglycerol in part by formation of angiotensin II (Ang II). However, the response of this pathway to physical stimuli in the adult heart is poorly understood. Thus, in isovolumic perfused guinea pig hearts, we characterized stretch-mediated phosphatidylinositol (PI) hydrolysis and protein kinase C (PKC) isoform translocation using elevated diastolic pressure. Balloon dilatation (minimum diastolic pressure, 25 mm Hg) of the left ventricle (LV) stimulated PI hydrolysis. Pretreatment of stretched hearts with the specific angiotensin (AT1) receptor antagonist losartan abolished stretch-mediated accumulation of inositol phosphates. To examine PKC isoform expression and activation under these conditions, whole-heart extracts were examined by immunoblot analysis. Ang II translocated PKC epsilon to the particulate fraction. 4 beta-Phorbol 12-myristate 13-acetate but not an inactive congener translocated PKC epsilon to the particulate fraction and produced a decrease in myocardial contractile function. Mechanical stretch also translocated PKC epsilon to the particulate fraction; however, this was attenuated but not abolished by losartan. We conclude that in the adult heart, LV dilation produced stretch-mediated activation of phospholipase C, which resulted in PI hydrolysis and PKC epsilon activation in part by stimulation of the local renin angiotensin system. In contrast to stretch-mediated inositol phosphate accumulation, PKC epsilon translocation is not prevented by AT1 receptor blockade, indicating that this PKC isoform can be activated in response to mechanical deformation by an Ang II-independent mechanism in the adult myocardium.
Paul et al. (Sat,) studied this question.