Positive Inotropy Mediated by Diacylglycerol in Rat Ventricular Myocytes

Abstract

Abstract Many neurohormones stimulate phospholipid hydrolysis and elevate diacylglycerol in the mammalian heart, but the physiological consequences of these intracellular events are unclear. Regulation of myocardial contraction by diacylglycerol was investigated in the present study by releasing the diacylglycerol analogue dioctanoylglycerol (diC 8 ) within adult rat ventricular myocytes by using a light-sensitive caged compound. This approach permitted us to avoid exposure of myocytes to extracellular diC 8 and yet to control the amount of diC 8 released into the cells. Photorelease of diC 8 produced a slowly developing (half-time, 1.9±0.1 minute; n=26) but robust (406±42%) enhancement of twitch amplitude in electrically paced myocytes (0.5 Hz, 1 mmol/L Ca 2+ Ringer’s solution [pH 7.4], 22°C). This positive inotropic effect was dose dependent, stereospecific for the S-enantiomer of diC 8 , synergistically enhanced by arachidonic acid, and blocked by the protein kinase C inhibitor chelerythrine. The data provide evidence that diacylglycerol can induce a strong positive inotropic effect in mammalian ventricular muscle, possibly by activating protein kinase C. By contrast, perfusion of diC 8 extracellularly onto myocytes caused a 42±2% decline in twitch amplitude, in accordance with previous reports. To account for this dependence on how diC 8 is applied, we postulate that diC 8 has distinct physiological actions at intracellular and extracellular sites. The peptide neurohormone endothelin-1, which elevates diacylglycerol in cardiac tissues, produced a positive inotropic effect that was similar to the response to photoreleased diC 8 . The diacylglycerol/protein kinase C pathway has now become a good candidate for mediator of at least a component of the positive inotropy associated with agents that stimulate phospholipid turnover in adult mammalian myocardium.