β-Adrenoceptor ligand binding and supersensitivity to isoprenaline of ventricular muscle after chronic reserpine pretreatment
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Isolate papillary muscles from guinea-pig hearts were paced at a constant frequency and isometric contractions recorded.
Guinea-pigs were either untreated or pretreated with reserpine. Three pretreatment schedules were used; a) 0.5 mg kg−1 i.p. at 24h, b) 5.0 mg kg−1 at 72h and 3.0 mg kg−1 at 48 and 24 h, or c) 0.1 mg kg−1 daily for 7 days.
Cumulative concentration-response curves for the isoprenaline-induced increases in tension were obtained. The geometric mean EC50 values after the 3 and 7 day reserpine pretreatment schedules were significantly (P<0.05) less than for untreated guinea-pigs indicating a supersensitivity.
EC50 values for the positive inotropic inotropic responses to histamine and calcium in papillary muscles from reserpine-pretreated guinea-pigs did not differ significantly (P<0.05) from those from untreated animals. This suggests that the supersensitivity to isoprenaline is β-adrenoceptor specific.
Membrane fractions were prepared from the ventricles of the untreated and reserpine-pretreated guinea-pigs from which papillary muscles had been removed. Binding of [3H]-dihydroalprenolol ([3H]-DHA) to β-adrenoceptors of these membranes was determined. Equilibrium dissociation constants (KD) and total numbers of binding sites (Bmax) were determined by Scatchard analysis of the saturation curves for [3H]-DHA binding.
There was no increase in affinity (fall in KD value) or change in the total number of binding sites associated with reserpine-induced supersensitivity. The equilibrium inhibition constant (Ki) for the displacement of [3H]-DHA binding by isoprenaline was also identical in membranes from untreated and reserpine-pretreated animals. Thus reserpine-induced supersensitivity to isoprenaline does not appear to involve a change in affinity for the β-adrenoceptor or in receptor numbers as determined by [3H]-DHA binding.
Key wordsCardiac β-adrenoceptors Reserpine Supersensitivity Papillary muscle [3H]-Dihydroalprenolol
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