The effect of melatonin (MLT) on the isolated rat heart was studied using the standard perfusion conditions (Langendorff preparation) and model of calcium paradox (Ca(2+)-paradox). Ca(2+)-paradox was induced by 1 minute perfusion with Ca(2+)-free Krebs-Henseleit (KH) solution and subsequent 20 minutes perfusion with a normal Ca(2+)-containing KH solution. In MLT group, MLT (10 micromol/l) was in the perfusion solution throughout the experiment. In controls, there was no MLT.
Variables: heart rate, coronary flow, systolic and diastolic pressure, +dP/dt max (index of contractility) and -dP/dt max (index of relaxation) were measured at the end of stabilization, i.e. after 30 minutes of standard perfusion and then in the 5th, 10th, 15th, 20th minute after perfusion with Ca(2+)-free KH solution.
Results: There was no difference between MLT group and controls in the standard perfusion conditions at the end of stabilization. After perfusion with Ca(2+)-free KH solution, systolic-diastolic difference (in the 10th, 15th, 20th minute), +dP/dt max (in the 5th, l0th, 15th, 20th minute) and -dP/dt max (in the 15th minute) were significantly decreased in MLT group in comparison to controls.
Conclusion: Melatonin didn't influence rat isolated heart in standard perfusion conditions but it made the heart more susceptible to Ca(2+)-paradox.