Adenosine 3',5'-cyclic monophosphate phosphodiesterase in guinea-pig lung—Properties and effect of adrenergic drugs

Abstract

The hydrolysis of adenosine 3',5'-cyclic monophosphate (cAMP) by adenosine 3',5'-monophosphate phosphodiesterase (PDE) was studied in whole homogenates and cell fractions of guinea-pig lung. Approximately 60 per cent of the activity of the whole homogenate was contained in the 10,000 g supernatant. This cell fraction contained PDE with two separate affinities for cAMP: PDE I with low affinity. K m ≅ 3.1 × 10 −4 M; V max ≅ 90 nmoles cAMP hydrolyzed/mg protein/20 min and PDE II with high affinity K m ≅ 5.7 × 10 −5 M; V max ≅ 35 nmoles cAMP hydrolyzed/mg protein/20 min. Both forms required magnesium which could be replaced by manganese and cobalt but not by copper. Aminophylline and theophylline inhibited both PDE forms. Calcium only inhibited PDE I but not PDE II. Adrenergic drugs inhibited PDE I and at 0.1 mM decreased in potency in the following order: isoproterenol  epinephrine > norepinephrine > methoxamine. Butoxamine was ineffective on PDE I. Only norepinephrine inhibited PDE II but not to the same extent as its effect on PDE I. Chelation of added magnesium and endogenous calcium was not responsible for the inhibition of PDE I by the adrenergic drugs used. Propranolol (0.1 mM) and phentolamine (0.05 to 0.001 mM) alone did not effect PDE I. Propranolol (0.1 mM) failed to reverse the inhibitory effect of isoproterenol (0.1 mM) and epinephrine (0.1 mM). Thus a β-adrenergic mechanism could not explain the inhibitory effect of these agents. Phentolamine (0.05 to 0.001 mM) reversed the inhibition by epinephrine bitartrate but not by epinephrine hydrochloride. These data are inconclusive to support an α-adrenergic mechanism for the inhibition of cAMP phosphodiesterase by epinephrine.