Abstract
Summary
The initial developed tension magnitude as well as the stability of the developed tension magnitude of isolated rat atria suspended in different substrates were studied. The initial developed tension was significantly greater in the presence of glucose, mannose, lactate acetate, butyrate, or pyruvate than when fructose or the nonmetabolizable monosaccharide 3-0-methylglucose was the substrate. Atrial developed tension in fructose was significantly greater than in 3-0-methylglucose or in substrate-free medium. The stability of the developed tension was comparable in 5.5 d glucose, mannose, lactate, acetate, butyrate, or pyruvate showing a small decrement with time (5-10% in 60-90 min period). Increasing concentrations of mannose, fructose, lactate, acetate, butyrate, or pyruvate up to 11.0 d did not prevent the decrement of developed tension whereas it was completely abolished in the presence of 11.0 d glucose. The ability of several substrates to modify the isometric contractile tension of rat atria suspended in KRB medium containing 5.5 mill glucose was also investigated. The addition of increasing concentrations of the metabolizable monosaccharides glucose and mannose, stimulate atrial developed tension, whereas fructose, had no effect. The 3-O-methylglucose had either no effect, or at higher concentrations resulted in a negative inotropic response. Increasing concentrations of butyrate, acetate, or pyruvate depressed the developed tension. It would appear that transport of sugars across the cell membrane or the increased formation of oxidizable substrates for the Krebs cycle are not ”per se” the only mechanism by which the metabolizable monosaccharides can maintain atrial developed tension or produce a positive inotropic effect. The possibility that the magnitude of atrial developed tension is influenced by some process that occur during the initial phases of the glycolytic sequence is discussed.
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