P2X-PURINOCEPTORS IN THE HEART - ACTIONS OF ATP AND UTP

Positive inotropic effects of ATP and UTP (1 mu M-1mM) were studied in isolated rat and guinea pig cardiac tissues. The potency order obtain ed was ATP>UTP in both species, suggesting possible interaction with P 2X-purinoceptors. Binding studies using [H-3]alpha,beta-methylene ATP as marker of P2X-purinoceptors revealed two receptor sites: one high-, the other low-affinity, in atria and ventricles from rat and guinea p ig. Both ATP and UTP were found to bind high-affinity sites of [H-3]al pha,beta-methylene ATP. The effects of various calcium inhibitors such as nifedipine, dantrolene, ryanodine and TMB-8 on positive inotropic effects induced by ATP and UTP were also studied. The results suggest that ATP and UTP may increase inotropism by interaction with P2X-purin oceptors by means of a calcium-dependent mechanism.