ADRENERGIC-RECEPTOR STIMULATION OF THE MITOGEN-ACTIVATED PROTEIN-KINASE CASCADE AND CARDIAC-HYPERTROPHY

Phenylephrine and noradrenaline (alpha-adrenergic agonism) or isoprena line (beta-adrenergic agonism) stimulated protein synthesis rates, inc reased the activity of the atrial natriuretic factor gene promoter and activated mitogen-activated protein kinase (MAPK). The EC(50) for MAP K activation by noradrenaline was 2-4 mu M and that for isoprenaline w as 0.2-0.3 mu M. Maximal activation of MAPK by isoprenaline was inhibi ted by the beta-adrenergic antagonist, propranolol, whereas the activa tion by noradrenaline was inhibited by the alpha(1)-adrenergic antagon ist, prazosin. FPLC on a Mono-Q column separated two peaks of MAPK (p4 2(MAPK) and p44(MAPK)) and two peaks of MAPK-activating activity (MEK) activated by isoprenaline or noradrenaline. Prolonged phorbol ester e xposure partially down-regulated the activation of MAPK by noradrenali ne but not by isoprenaline. This implies a role for protein kinase C i n MAPK activation by noradrenaline but not isoprenaline. A role for cy clic AMP in activation of the MAPK pathway was eliminated when other a gonists that elevate cyclic AMP in the cardiac myocyte did not activat e MAPK. In contrast, MAPK was activated by exposure to ionomycin, Bay K8644 or thapsigargin that elevate intracellular Ca2+. Furthermore, de pletion of extracellular Ca2+ concentrations with bis-(o-aminophenoxy) ethane-NNN'N'-tetraacetic acid (BAPTA) or blocking of the L-type Ca2channel with nifepidine or verapamil inhibited the response to isopren aline without inhibiting the responses to noradrenaline. We conclude t hat alpha- and beta-adrenergic agonists can activate the MEK/MAPK path way in the heart by different signalling pathways. Elevation of intrac ellular Ca2+ rather than cyclic AMP appears important in the activatio n of MAPK by isoprenaline in the cardiac myocyte.