α1-Adrenoceptors of rat myocardium: Comparison of agonist binding and positive inotropic response
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At 30°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 I738 nmol/l). GTP transformed all binding sites into the low affinity form suggesting that at least some of the cardiac α1-adrenoceptors are coupled to N-proteins.
At 0°C most of the binding sites (86%) were in a state of high affinity for agonists (KI for adrenaline: 91 nmol/l).
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 al-binding sites.
When temperature was changed during incubation with adrenaline, a rise of temperature (from 0°C to 30°C) converted high affinity sites into the low affinity form, whereas a decrease in temperature (from 30°C to 0°C) failed to induce the high affinity state for agonists. Short term incubation (0.5 min) with adrenaline at 30°C resulted in significantly lower IC50 values as compared to equilibrium conditions at the same temperature.
Occupancy of al-adrenoceptor binding sites with adrenaline at O°C but not at 30°C rather closely paralleled the concentration-response curve for the α1-mediated increase in ontractile force.
Irreversible blockade of α1-adrenoceptors with phenoxybenzamine decreased the maximum inotropic response but only slightly affected pD2 values for the α1-stimulant effect of adrenaline.
Our binding experiments suggest that myocardial α1-adrenoceptors can exist in states of different affinity for agonists. Some agonists like adrenaline and phenylephrine seem to induce a temperature-dependent change in the conformation of the receptor which may represent a rapid form of desensitization. Since no appreciable receptor reserve was detected it is hypothesized that the high affinity state which can be measured at 0°C under equilibrium conditions may be relevant to the initiation of the functional response.
Key wordsCardiac α1-adrenoceptors 3H-Prazosin binding Positive inotropic effect Temperature sensitivity Receptor desensitization
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- Barlow RB (1983) Biodata handling with microcomputers. Elsevier, AmsterdamGoogle Scholar
- Benfey MD (1987) Function of myocardial α-adrenoceptors. J Appl Cardiol 2:49–70Google Scholar
- Contreras ML, Wolfe BB, Molinoff PB (1986a) Thermodynamic properties of agonist interactions with the beta-adrenergic receptor-coupled adenylate cyclase system. I. High- and low-affinity states of agonist binding to membrane-bound β-adrenergic receptors. J Pharmacol Exp Ther 237:154–164PubMedGoogle Scholar
- GrosB G, Hanft G (1988) 5-Methyl-urapidil — an antagonist which discriminates between α1-adrenoceptor subtypes. Br J Pharmacol, Proceedings Suppl (in press)Google Scholar