Epinephrine regulation of phosphofructokinase in perfused rat heart. A calcium ion-dependent mechanism mediated via alpha-receptors.

A detailed examination of the previously reported a-adrenergic activation of heart muscle phosphofructokinase (Clark, M. G., and Patten, G. S. (1981) Nature 292, 461-463) was made. The a-agonist, naphazoline, increased the activity ratio of phosphofructokinase and decreased the intracellular concentration of hexose 6phosphate but had no significant effect on the activity ratio of phosphorylase and the rate of beating. Agonists with both a and /3 activity (epinephrine, norepinephrine, and phenylephrine) increased beating, the concentration of hexose &phosphate, and the activity ratios of phosphofructokinase and phosphorylase. The two agonists with predominantly /3 activity, isoproterenol and ritodrine, increased the rate of beating and the concentration of hexose 6-phosphate and partly increased the activity ratio of phosphofructokinase. Halfmaximal activation of phosphofructokinase, assessed by the change in the activity ratio, occurred at 10 nM epinephrine, 10 m norepinephrine, and 100 m naphazoline. Half-maximal activation of phosphorylase occurred at 500 nM epinephrine and 50 nM norepinephrine. Activation of phosphofructokinase by 0.5 PM epinephrine was unaffected by propranolol but was substantially blocked by 10 p~ phenoxybenzamine. The role of Ca2+ ions in the activation of phosphofructokinase was examined. At low concentrations of Ca2+ ions (<0.1 mM), 10 p~ epinephrine activated phosphorylase but did not activate phosphofructokinase. There was no indication of a direct interaction between Ca2+ ions and the activated form of phosphofructokinase. Furthermore, neither trifluoperazine (140 m) nor ethylene glycol bis(P-aminoethyl ether)-N,iVflfl-tetraacetic acid (0.6 mM) accelerated the decay of the activity ratio of the activated enzyme in heart extracts at 0 “C. It is concluded that phosphofructokinase in heart is regulated in the short erm by catecholamines acting through either aor P-adrenergic receptors. For epinephrine (0.5 p ~ ) , the mechanism of activation of phosphofructokinase appears to be Ca2+-dependent and mediated predominantly through a-adrenergic receptors.