Investigation of effects of positive inotropic agents on day-night difference of Myosin Light Chain activations in isolated rat heart

Experiments were performed at two different time points (6 HALO and 18 HALO). After ouabain (10-4, 10-5, 10-6 and 10-7 M) or dobutamine (10-4, 10-5 and 10-6 M) perfusion ventricular tissues were quickly freezed for preparation of tissue homogenates. The day versus night difference in myosin light chain expression and activation levels were determined by Western Blot method.Wistar rats were anesthetized with thiopental sodium and isolated rat hearts mounted on Langendorff apparatus immediately. Left ventricular pressure was measured with a water-filled baloon. After a 20 minute stabilisation period hearts perfused with each concentration of drugs for 10 minutes. At the end of drug perfusion hearts were taken off and immersed in liquid nitrogen. Myosin light chain-1, myosin light chain-2, actin expressions and myosin light chain activations were evaluated in ventricular tissues by using Western Blot method.It was shown that basal contractility was significantly higher at night but there was not significant difference between day and night levels of myosin light chain-1,myosin light chain-2, phospho myosin light chain-2 and actin expressions.Dobutamine did not lead to a significant difference between day and night heart contractility. On the other hand, after 10-5 M ouabain perfusion, the increase in left ventricular developed pressure was higher in day group compared to night group. Besides, level of myosin light chain phosphorylation was not altered by dobutamine and ouabain perfusion in day versus night groups.