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The main objective of the present study was the examination of the influence of PKC activation on cardiac contractility and Ca2+ handling by modulating the B56α-regulated PP2A activity. For this purpose PKC was activated by the α-adrenoceptor agonist phenylephrine (PE) and phorbol 12-myristate 13-acetate (PMA) in ventricular cardiomyocytes and left atria of B56α-overexpressing and wild-type mice. The functional effects of pharmacological PKC activation by PE and PMA were determined by measuring myocellular sarcomere lengths (SL) and Ca2+ transients as well as force of contraction on left atrial preparations. The application of PE to wild-type cells, reflecting native physiological conditions, resulted in an increase of the peak amplitude and a reduction of the decay time of Ca2+ transients. SL shortening and relaxation time were unchanged compared to controls. These findings might be explained by a decrease of the myofilament Ca2+ sensitivity. The direct activation of PKC by PMA caused an increase in SL shortening and Δ[Ca]i in wild-type cardiomyocytes, both in the presence or absence of PE. Furthermore, relaxation time and Ca2+ transient kinetics were shortened. We observed comparable contractile effects after stimulation with PMA and PE in left atria. In contrast to wild-type cells, transgenic cardiomyocytes exhibited parallel increases
of myocellular contractility and peak amplitude of Ca2+ transients after PKC stimulation. These effects may result from a higher basal myofilament Ca2+ ensitivity in transgenic cardiomyocytes. The increase in SL shortening and Δ[Ca]i in wild-type cardiomyocytes, following maximum PKC activation by PMA (± PE), was associated with a decrease in PP2A activity in extracts of isolated cells or atrial preparations, whereas PP1 activity was not changed. In transgenic samples, PP2A and PP1 activities remained unchanged after PMA application (± PE). However, the reduction in PP2A activity in wild-type hearts was not accompanied by changes in the total phosphorylation status of the regulatory subunit B56α. This might result from either a phosphorylation of a different regulatory B56 subunit or other B56α-independent mechanisms. Taken together, according to the underlying working hypothesis this study provides evidence for a relationship between PKC activation, decreased PP2A activity and positive inotropy in myocardial preparations. However, the reduced PP2A activity is not a direct consequence of an increased B56α phosphorylation under experimental conditions of the present study.