Short‐Term Effects of Calcium, Potassium, and of Ouabain on Metabolite Levels in the Frog Heart in vivo

The short‐term effects of calcium (7.2 mM), potassium (10.7 mM), and of ouabain (0.1%) have been studied in the frog heart in vivo . No dynamic alterations occurred at 30 sec, whereas positive inotropism was observed at 60 sec in the high‐Ca and ouabain treatment; in all cases heart arrest occurred at 120 sec. 18 metabolites (glucose, glucose‐1‐ P , glucose‐6‐ P , fructose‐6‐ P , fructose‐1,6‐ P 2 , dihydroxyacetone‐ P , glyceraldehyde‐3‐ P , α‐ P ‐glycerol, pyruvate, lactate, oxaloacetate, malate, α‐ketoglutarate, P ‐creatine, creatine, ATP, ADP, AMP) and 17 added parameters (mass‐action ratios of the phosphoglucomutase, phosphohexose isomerase, triosephosphate isomerase, phosphofructokinase, aldolase, adenylate kinase, creatine kinase; redox ratios; ratios P ‐creatine/creatine, ATP/ADP; sums P ‐creatine + creatine, ATP + ADP + AMP, lactate + pyruvate) were computerized by the stepwise discriminant analysis. Highly significant discrimination between control and treated hearts (thanks to the sensitivity of the statistical method employed) shows that ionic shifts and ouabain elicited modifications of the metabolite pattern which always preceded the effects on heart contractility. The most meaningful results were as follows:1 High calcium effect: At 30 sec, the phosphofructokinase‐catalyzed step and the pyruvate, α‐ P ‐glycerol and oxaloacetate‐depleting reactions are activated; the P ‐creatine and ATP balance is negative; ADP and AMP are increased. At 60 and 120 sec the phosphorylase system, phosphofructokinase, and the whole glycolytic sequence are activated. 2 High‐potassium effect: At 30 sec the phosphofructokinase‐step and the glycolytic flux are activated. Glucose entry into the cell is impaired. P ‐creatine, ATP, AMP, ATP/ADP, P ‐creatine/creatine are increased. At 60 sec, all glycolytic substrates are increased. 3 Ouabain effect: At 30 sec glucose extraction from the perfusing fluid decreases, lactate, pyruvate, α‐ P ‐glycerol and malate accumulate; P ‐creatine and ATP are increased. At 60 sec there is no evidence for activation of the phosphorylase system; the glycolytic pattern points to activation of glucose extraction and of the glucose shunt, and to inhibition of the glyceraldehyde‐3‐ P dehydrogenase and 3‐ P ‐glycerate kinase steps. P ‐creatine, ATP, and AMP are decreased, creatine and ADP increased. The partial overlapping of the high‐calcium and ouabain patterns suggests that the glycoside not only raises the intracellular free‐calcium level but also depresses the glyceraldehyde‐3‐ P dehydrogenase, 3‐ P ‐glacerate kinase and adenylate cyclase activity. The observed effects are only partially in accord with the inhibition of Na + ‐K + ‐dependent adenosinetriphosphatase.