Regulation of A₁ adenosine receptors by amiodarone and electrical stimulation in rat myocardial cells in vitro

The effects of conditions that either increase or decrease heart rate on the pharmacological properties of adenosine receptors in cultured rat myocytes were examined. Levels of A₁ adenosine receptors, following prolonged treatment with electrical stimulation (ES) or the antiarrhythmic drug amiodarone, were determined using radioligand binding with the specific A₁ receptor antagonist [³H]1,3-dipropyl-8-cyclopentylxanthine (CPX). The effects of lowering temperature were also explored. Exposure to amiodarone for 4 days reduced the density of A₁ receptors by 19% (from 24.7 ± 0.4 to 20.09 ± 0.3 fmol/dish) and inhibited the rate of contraction by 60% (from 188 ± 16 to 76 ± 30 beats/min), without changing the receptor affinity, protein content, creatine kinase (CK) activity or cell number. Electrical stimulation at 25°C elevated the density of A₁ adenosine receptors by 185% (from 4.1 ± 0.4 to 11.69 ± 2.1 fmol/dish). Four days of reduced temperature (from 37°C to either 30 or 25°C) lowered the density of A, adenosine receptors by 69 or 86%, respectively (from 24.1 ± 1.2 to 7.4 ± 0.4 or 3.4 ± 0.3 fmol/dish), with no significant change in the receptor affinity, activity of CK, or lactate dehydrogenase (LDH), protein content or cell number. The observed up- and down-regular-ion of A₁ adenosine receptors in primary myocyte cultures in response to conditions that exogenously alter the rate of contraction, is indicative of the role of adenosine receptors in adaptation of heart cells to stress.