Alpha 1-adrenoceptors of Rat Myocardium: Comparison of Agonist Binding and Positive Inotropic Response

Binding of agonists to alpha 1-adrenoceptors labelled by 3H-prazosin was investigated in membranes of rat myocardium and compared to the inotropic response elicited by alpha 1-adrenoceptor activation on isolated right ventricles. 1. At 30 degrees C the full agonists, adrenaline and phenylephrine, displaced 3H-prazosin with a shallow inhibition curve. The data are compatible with the assumption that 32% of the binding sites were in a state of high affinity for the agonist adrenaline (KI 85 nmol/l) and 68% in a low affinity state (KI 1738 nmol/l). GTP transformed all binding sites into the low affinity form suggesting that at least some of the cardiac alpha 1-adrenoceptors are coupled to N-proteins. 2. At 0 degree C most of the binding sites (86%) were in a state of high affinity for agonists (KI for adrenaline: 91 nmol/l). 3. For several partial agonists and antagonists (cirazoline, methoxamine, indanidine (Sgd 101-75), oxymetazoline and phentolamine) no such distinct temperature- and GTP-shifts could be demonstrated suggesting a different kind of interaction with alpha 1-binding sites. 4. When temperature was changed during incubation with adrenaline, a rise of temperature (from 0 degrees C to 30 degrees C) converted high affinity sites into the low affinity form, whereas a decrease in temperature (from 30 degrees C to 0 degrees C) failed to induce the high affinity state for agonists. Short term incubation (0.5 min) with adrenaline at 30 degrees C resulted in significantly lower IC50 values as compared to equilibrium conditions at the same temperature.(ABSTRACT TRUNCATED AT 250 WORDS)