DYNAMIC CHANGES OF MYOCARDIAL INOSITOLTRISPHOSPHATE AND CYCLIC-NUCLEOTIDES - RELATIONSHIP TO CONTRACTILE RESPONSE IN THE PERFUSED WORKING RAT-HEART AFTER ADRENERGIC AND MUSCARINIC AGONIST STIMULATION

Initial and late effects by adrenergic and muscarinic agonists on inositol (1,4,5) trisphosphate (IP3) and cyclic nucleotide levels were determined and correlated to mechanical response in perfused rat hearts. Forty-three rat hearts were perfused with Krebs-Henseleit buffer in a modified Langendorff apparatus as a working preparation. The hearts were perfused as controls (n = 11), or with noradrenaline (10(-6) mol l(-1)) (n = 21), or with carbachol (3 x 10(-7) mol l(-1)) (n = 11) added to the perfusion buffer. The hearts were frozen at 20 s, 30 s and 40 min after addition of noradrenaline and at 20 s and 40 min after addition of carbachol, and after 5 and 45 min of control perfusion. cAMP and cGMP were determined by radioligand methods and IP3 by a combined fast performance liquid chromatography (FPLC)- isotachophoretic method. cAMP increased by 36% within 20 s followed by a decrease (22%) during the 10 s following noradrenaline addition. After 40 min cAMP regained its value near that of 20 s. Noradrenaline perfusion did not influence IP3 levels during the first 30 s although the value at 40 min was significantly higher (59%). IP3 increased (42%) after 20 s of carbachol perfusion followed by a 25% decrease at 40 min. Sustained stimulation of beta-receptors (after 40 min in our model) resulted in a repeated increase in cAMP only, without an increase in contractility. Muscarinic receptor stimulation resulted in an early increase in IP3 content without a change in mechanical function whereas alpha-adrenergic stimulation resulted in only a late increase in IP3 content of isolated whole hearts still without any alteration in contractility.