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The pulmonary artery responses in the isolated whole-blood perfused canine lung to ionized calcium ([Ca++]) were quantified over a range of hypercalcemia and hypocalcemia values ([Ca++] = 0.23–1.88 mM) under conditions of controlled pulmonary blood flow and constant mean aortic and left atrial pressures. Calcium chloride, administered as bolus doses in the clinical range (5–15 mg.kg-1) at initial normocalcemia and without interventions producing vaso-constriction did not influence mean pulmonary artery pressure at constant pulmonary blood flow. Stable hypercalcemia ([Ca++] = 1.88 ± 0.05 mM) did not influence the slope of the pulmonary artery pressure-flow plot. Because normal pulmonary vasomotor tone is low and cannot readily be lowered further, the possible vasodilator action of hypocalcemia was assessed by its ability to decrease the slope of the mean pulmonary artery pressure-flow plot, which had been first increased by alveolar hypoxia (AHX) or infusion of the prostaglandin endoperoxide analog U46619 (PG). During AHX (n = 5), a graded reduction from normocalcemia ([Ca++] = 1.08 ± 0.02 mM) to moderate hypocalcemia ([Ca++] = 0.8 and 0.5 mM) did not alter the pulmonary artery pressure-flow plot, but severe hypocalcemia ([Ca++] = 0.26 ± 0.01 mM) decreased the slope by 13 ± 0.9 mmHg.1-1.min-1. The comparison of severe hypocalcemia ([Ca++] = 0.23–0.27 mM) versus a high dose of nifedipine (bolus of 10 μg/kg followed by continuous infusion at 40 μg.kg-1.h-1) on pulmonary vascular tone increased by either AHX or PG infusion indicated that both hypocalcemia and nifedipine decreased the slope of the relationship between mean pulmonary artery pressure and flow (during AHX: −16.1 ± 1.38 and −23.3 ± 1.73 mmHg.1-1.min-1, both P = 0.0001 vs. AHX alone, and during PG: −17.05 ± 1.95 and −8.4 ± 1.78 mmHg.1-1.min-1, P = 0.0001 vs. PG alone). Two principal conclusions emerge. First, the pulmonary vessels are minimally sensitive to changes in ionized calcium throughout the clinical hypercalcemia and hypocalcemia ranges; extreme hypocalcemia is required to produce vasodilation, which was reversed with calcium infusion. Second, whereas the pulmonary vasodilator effects of extreme hypocalcemia were independent of the intervention inducing pulmonary vasoconstriction (AHX vs. PG), those of nifedipine were much more pronounced with AHX.