Effects of cyclic AMP on the function of the cardiac gap junction during hypoxia

In the ischemic or hypoxic heart, an impairment of electrical cell-to-cell coupling and a dephosphorylation of the connexins that comprise the gap junction channel were observed. However, it remains to be elucidated whether the dephosphorylation of the connexin during hypoxia is due to alterations in the ionic strength of Ca(2+) or H(+), and how the activation of protein kinase A (PKA) affects the hypoxia-induced abnormal function of the gap junction.

The effects of hypoxia, intracellular Ca(2+) overload and intracellular acidosis on the PKA-mediated phosphorylation of connexin 43 (Cx43) were examined in relation to the function of the cardiac gap junction.

Hearts isolated from adult, male guinea pigs were used. The intercellular electrical cell-to-cell coupling was evaluated by the longitudinal internal resistance and the conduction velocity observed in in vitro experiments using isolated muscle strip preparations. The phosphorylation of Cx43 was evaluated by an immunoblot (Western blot). The localization of immunoreactive Cx43 at the intercalated disk was detected using confocal laser scan microscopy.

Cyclic AMP or the activation of PKA promotes the intercellular electrical coupling that accompanies an augmentation of the PKA-mediated phosphorylation of Cx43. Electrical cell-to-cell decoupling and reduction of the PKA-mediated phosphorylation of Cx43 were dependent on the progression of hypoxia. These results agree with those observed in the progression of intracellular Ca(2+) overload or intracellular acidosis. Cyclic AMP or the activation of PKA alleviated the electrical cellular decoupling and the hypoxia-, intracellular Ca(2+) overload- and intracellular acidosis-induced deteriorated expression of Cx43. These ameliorative effects of cyclic AMP on the function of the gap junction and on the expression of Cx43 weakened as the hypoxia progressed, and as the intracellular ionic strength of Ca(2+) and H(+) increased.

In cardiac ventricular muscle cells, cyclic AMP or the activation of PKA promotes electrical cell-to-cell coupling through the gap junction due to an augmentation of the PKA-mediated phosphorylation of Cx43 in the early stage of hypoxia, as well as in normoxia. The suppression of PKA-mediated phosphorylation of Cx43 during hypoxia may be caused by an increase in the intracellular ionic strength of Ca(2+) and H(+). Thus, the activation of cyclic AMP-dependent PKA may have an antiarrhythmic effect in the early stage of hypoxia.