α1‐ and β‐Adrenoceptor‐Mediated Increase in86Rb+‐Uptake in Isolated Cardiomyocytes from Adult Rat Heart: Evidence for Interaction between the Two Receptor Systems

The aim of the present investigation was to study the effect of alpha(1)- and beta-adrenoceptor stimulation, alone and in combination, on potassium uptake in isolated ventricular cardiomyocytes from adult rat heart, using the potassium analogue 86Rb+. The reliability of 86Rb+ as a potassium analogue was also investigated. Alpha(1)-, beta-, And combined adrenoceptor stimulation was achieved by using noradrenaline in the presence and absence of appropriate adrenoceptor antagonists. The uptake of 86Rb+ was found to increase linearly with time up to 20 min., both during basal and receptor-stimulated conditions. The basal uptake rate was about 0.18 ml/g protein x min. At 15 min. both alpha(1)-, beta- and combined adrenoceptor stimulation dose-dependently increased the 86Rb(+)-uptake with a -logEC50 of 7.05, 6.68 and 6.73, respectively. The maximal increase in these series achieved by 5 x 10(-5) mol/l noradrenaline was 29%, 24% and 41% above basal level, respectively. Comparison of the maximal effects in the same cell preparations, with the observed value for combined adrenoceptor stimulation in each experiment as 100%, gave a relative maximal increase in 86Rb(+)-uptake after separate alpha(1)-adrenoceptor stimulation of 67 +/- 8%, and of 68 +/- 6% after separate beta-adrenoceptor stimulation. The theoretically calculated value for combined adrenoceptor stimulation, if additivity, was 135 +/- 11%, which was significantly higher than the observed value (100%) (P = 0.026). The effect of noradrenaline was not limited by the maximal 86Rb(+)-uptake capacity, as 10(-5) mol/l forskolin increased the 86Rb(+)-uptake more than noradrenaline. Examining the reliability of 86Rb+ as potassium-analogue by combining 42K+ and 86Rb+ in the same experiments, showed that combined adrenoceptor stimulation dose-dependently increased both the 42K(+)- and 86Rb(+)-uptake with the same potency and to the same extent. Thus 86Rb+ is a reliable potassium-analogue for these effects. In conclusion both alpha(1)- and beta-adrenoceptor stimulation dose-dependently increased the cellular 86Rb(+)-uptake to the same extent and with the same potency. The observed maximal 86Rb(+)-uptake after combined adrenoceptor stimulation was significantly higher than the maximal effect after either form of separate receptor stimulation, but significantly lower than expected if the effects were purely additive. The results thus show inhibitory interaction between the two receptor systems.