Differences in the association of calmodulin with cyclic nucleotide phosphodiesterase in relaxed and contracted arterial strips

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Abstract
Changes in the concentration of cytosolic Ca2+ are assumed to alter the activity of Ca2+-calmodulin-sensitive cyclic nucleotide phosphodiesterase in intact cells. This assumption is based on indirect evidence and by analogy from studies of enzyme activities in broken cell systems. A procedure was developed for estimating the fraction of Ca2+-calmodulin-sensitive phosphodiesterase that is in an activated, ethylene glycol bis(.beta.-aminoethyl-ether)-N,N,N'',N''-tetraacetic acid (EGTA) sensitive state in intact porcine coronary artery strips. The experimental approach involves homogenization of the strips and assay of cGMP phosphodiesterase activity under conditions that retard changes in the amount of the complex Ca2+-calmodulin-phosphodiesterase. cGMP phosphodiesterase in intact coronary artery strips does associate with Ca2+-calmodulin, and interventions that change the concentration of Ca2+ in the cytosol of the intact strip change the extent of this functional association. Exposure to histamine (10 or 100 .mu.M) or 50 mM KCl caused contraction and an increase in EGTA-sensitive cGMP phosphodiesterase activity. Isoproteronol-induced relaxation of tissues that had been caused to contract with 10 .mu.M histamine was accompanied by a reduction in EGTA-sensitive cGMP phosphodiesterase activity to the same level as that present before contraction was initiated.