Polytechnic, Leicester LE1 9BH 1 Rat isolated and superfused atria weie exposed to a lactate-containing solution simulating the composition of extracellular fluid during myocardial ischaemia (SI). 2 Atria subjected to SI showed a decreased adenosine 5'-triphosphate (ATP) content, a rise in diastolic tension, a diminished conduction velocity of action potentials and shortened refractory periods. All these changes were less pronounced during lactate-free SI. 3 Atria preloaded with calcium displayed exaggerated responses measured electrically and mechanically during exposure to SI, whereas atria previously depleted of calcium displayed diminished electrical and mechanical responses to SI. Neither calcium loading nor calcium depletion modified the SI-induced depletion of the atrial stores of ATP. 4 Sulphinpyrazone protected atria against all aspects of the response to SI, but failed to protect the muscle under conditions of lactate-free SI. It is concluded that during SI, sulphinpyrazone protects against a lactate-mediated inhibition of the glycolytic synthesis of ATP. 5 Flufenamate exaggerated all responses of the atria to SI. These deleterious actions were still observed during lactate-free SI. It is concluded that flufenamate inhibits the synthesis of ATP in the mitochondria.
IntroductionSimulated ischaemia (SI) is said to exist when a tissue is exposed in vitro to a solution the composition of which resembles that found extracellularly in regions of ischaemia. During SI in rat atrial muscle the conduction velocity (CV) of action potentials is reduced, as are the durations of both action potentials and refractory periods. These responses are accompanied by progressive contracture development and a characteristic pattern of structural changes, notably in the mitochondria (Northover & Northover, 1988). Return of the myocardium to a solution of normal composition gradually reverses all of these changes. Pretreatment of the atria with sulphinpyrazone protects the myocardium during SI, but flufenamate, another non-steroidal antiinflammatory agent, has a deleterious action (Northover & Northover, 1988).A rise in the diastolic concentration of Ca in the myoplasm is believed to represent an important stage in the series of events responsible for the structural and functional changes observed during SI (Northover & Northover, 1988). In some of the present experiments the diastolic Ca concentration in the myoplasm has been deliberately elevated before exposure to SI. By noting which components of the response to SI were enhanced under these circumstances evidence was obtained concerning the ways in which Ca is involved as a mediator.Lactate is another substance known to accumulate in the ischaemic myocardium (Neely & Grotyohann, 1984), and is an ingredient of the fluid used to produce SI. Moreover, lactate is known to potentiate some of the functional changes in the atria produced by the other ingredients of this fluid (Northover, 1987). Under certain circumstances lactate is known to inhibit glycolysis (Rovetto et al...