Response of cardiac myocytes to a ramp increase of diacylglycerol generated by photolysis of a novel caged diacylglycerol.

To test the responsiveness of living cells to the intracellular messenger diacylglycerol, we developed a prototype caged diacylglycerol compound, 3-O-(alpha-carboxyl-2,4-dinitrobenzyl)-1 ,2-dioctanoyl-rac-glycerol (designated alpha-carboxyl caged diC(8)), that produces dioctanoylglycerol (diC(8)) on photolysis. Alpha-Carboxyl caged diC(8) is biologically inert toward diacylglycerol kinase and protein kinase C in vitro and is readily incorporated into cardiac myocyte membranes, where it has no effect before irradiation. Exposure to near-UV light releases biologically active diC8 in good yield (quantum efficiency = 0.2). Here we examine a cellular response to controlled elevation of diC8 within single cardiac myocytes. Twitch amplitude was monitored in electrically stimulated myocytes, and a ramp increase in the concentration of diC(8) was generated by continuous irradiation of cells loaded with the caged compound. The myocyte response was biphasic with a positive inotropic phase (39% increase in twitch amplitude), followed by a large negative inotropic phase (>80% decrease). The time to peak inotropy for both phases depended on the light intensity, decreasing from 376 +/- 51 S to 44 +/- 5 s (positive phase) and 422 +/- 118 S to 51 +/- 9 S (negative phase) as the light intensity was increased eightfold. Both phases were inhibited by the protein kinase C inhibitor chelethyrine chloride. An increase in extracellular K+ from 5 mM to 20 mM to partially depolarize the cell membrane eliminated the positive inotropic phase, but the negative inotropic response was largely unaltered. The results reveal new features in the response of cardiac muscle to diacylglycerol, including a positive inotropic phase and a complex responsiveness to a simple linear increase in diacylglycerol. The effects of photoreleased diC(8) were similar to the effects of opiate agonists selective for kappa receptors, consistent with a major role for diacylglycerol in these responses.