Dependence on Ca2+ of the activities of phosphatidylinositol 4,5-bisphosphate phosphodiesterase and inositol 1,4,5-trisphosphate phosphatase in smooth muscles of the porcine coronary artery

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Abstract
The activities of phosphatidylinositol 4,5-bisphosphate (PIP2) phosphodiesterase (PDE) and inositol 1,4,5,-trisphosphate (IP3) phosphatase in the particulate and cytosol fractions prepared from porcine coronary artery smooth muscles were examined using 32P-labelled PIP2 and IP3 as substrates, respectively. The activity of PIP2 PDE, as assessed from the production of IP3, in the cytosol fraction was about 10-fold higher than that in the particulate fraction. In the absence of MgCl2, the activity of PIP2 PDE in both fractions showed no causal relation to the free Ca2+ concentration in the physiological range of 10(-7)-10(-5) M, but was enhanced remarkably by 10(-4) M free Ca2+. The addition of 1 mM-MgCl2 to the assay medium markedly inhibited the activity of PIP2 PDE in both fractions in the presence of free Ca2+ (10(-8)-10(-5) M). In the absence of MgCl2, 10(-5)M-acetylcholine (ACh) produced IP3, and this action was blocked by 3 × 10(-6) M-atropine. The ACh-induced activation of PIP2 PDE ceased in the presence of 1 mM-MgCl2; however, the reactivation occurring on the addition of 10 microM-guanosine 5′-[γ-thio]triphosphate did not depend on the free Ca2+ concentrations (10(-7)-10(-5)M). The activities of IP3 phosphatase, determined from decrease in the amount of IP3 in the particulate and cytosol fractions, had much the same potency in both fractions. The activity of IP3 phosphatase in the cytosol fraction was enhanced by MgCl2 in a concentration-dependent manner, the maximal value occurring at 1 mM-MgCl2, and was also enhanced in the presence of physiological concentrations of free Ca2+ (10(-7)-10(-6) M). These findings suggest that the activation of PIP2 PDE which occurs with application of ACh in the presence of guanine nucleotides and 1 mM-MgCl2 is independent of the free Ca2+ concentration, and that the hydrolysis of IP3 by phosphatase increases, depending on the concentration of free Ca2+.