EFFECTS OF VASODILATOR DRUGS, ALKALINE PHOSPHTASE, AND CYCLIC AMP‐DEPENDENT PROTEIN KINASE ON THE 45CALCIUM UPTAKE OF SARCOLEMMAL MICROSOMES FROM HUMAN UMBILICAL ARTERIES

Abstract

1 A microsomal fraction was prepared from human umbilical arteries by differential centrifugation. The preparation was capable of an oxalate-stimulated Ca²⁺ uptake at a mean rate of 0.74 nmol Ca²⁺ mg⁻¹ protein min⁻¹ which could be inhibited by a Ca²⁺ ionophore, A 23 187, and by Tween 80. 2 The rate of Ca²⁺ uptake in the fractions obtained by density gradient fractionation paralleled 5′-nucleotidase activity suggesting that vesicles of predominantly sarcolemmal origin were responsible for the microsomal Ca²⁺ uptake. 3 Cyclic adenosine 3′,5′-monophosphate-dependent protein kinase enhanced membrane phosphorylation but did not affect Ca²⁺ uptake. Preincubation with alkaline phosphatase reduced membrane phosphorylation to a greater extent than Ca²⁺ uptake. These data are not in favour of a close correlation between Ca²⁺ uptake and phosphorylation. 4 None of 15 vasodilator drugs (bencyclane, carbocromen, diazoxide, dilazep, hydralazine, indapamide, isosorbide dinitrate, methyl-isobutyl-xanthine, minoxidil, naftidrofuryl, nitroglycerine, prenylamine, sodium nitroprusside, tetracaine, and verapamil) had any effect on Ca²⁺ uptake at 10⁻⁵ m. This suggests that vasodilator drugs do not act by a direct influence on the Ca²⁺ pumps of vascular smooth muscle cells.