INFLUENCE OF ISCHEMIA AND REPERFUSION ON CARDIAC SIGNAL-TRANSDUCTION - G-PROTEIN CONTENT, ADENYLYL-CYCLASE ACTIVITY, CYCLIC-AMP CONTENT, AND FORSKOLIN AND DIBUTYRYL CYCLIC AMP-INDUCED INOTROPY IN THE RAT LANGENDORFF HEART

We investigated whether post-receptor alterations contribute to the di minished beta-adrenergic inotropic effects in the rat Langendorff hear t following ischaemia (I) and reperfusion (R). We quantitated immunode tectable G(s) and G(i) protein alpha-subunit content, basal and stimul ated adenylyl cyclase activity and cyclic AMP (cAMP) content in normox ic, ischaemic (30 min) and ischaemic reperfused (30 min) hearts. In ad dition, we measured the inotropic response of normoxic and reperfused Langendorff hearts to forskolin and dibutyryl cAMP (db-cAMP). Immunode tectable G(s) and G(i) alpha-subunits were unaltered by I or R. Basal adenylyl cyclase activity was decreased during I, but recovered during R. In membranes from normoxic hearts, isoprenaline, GTP, Gpp(NH)p, Na F, forskolin or Mn2+ enhanced adenylyl cyclase activity. This increase in activity was diminished in ischaemic hearts, but could be restored by R. cAMP content decreased time-dependently during I and did not re cover by R, indicating ATP depletion. Forskolin and db-cAMP induced an inotropic response in normoxic hearts, which was virtually abolished after I and R. We conclude that adenylyl cyclase responsiveness is imp aired during I. Since adenylyl cyclase responsiveness recovers during R, whereas inotropic responses to forskolin and db-cAMP are virtually absent in reperfused hearts, an additional mechanism downstream of cAM P formation appears to be defective during R, which prevents recovery of inotropic responses to hormonal stimulation.