Comparison of propranolol and metoprolol on isolated frog heart

Abstract

Sympathomimetic drugs and drugs that block adrenoceptors have important effects, some of which are of great clinical value. These effects vary dramatically according to the drug's selectivity for  and  receptors.  1 and  2 adrenoceptors coexist in the heart of various animal species, including man. Competitive radio-ligand binding studies performed in membranes from homogenized hearts have shown that only 20-30% of the total β-adrenoceptors are of the β 2-subtype in adult mammalian ventricular tissue. This number is even further reduced when purified cardiac myocytes rather than homogenized tissues are used. Unlike the mammalian heart, β-adrenoceptor population in the frog heart is composed of a majority (~ 80%) of β 2-receptors. Frogs, weighing 150-250g, double pith a frog and fasten it to a frog board, ventral side up, midline incision was given on the abdomen. Pectoral girdle was removed and the heart was exposed, Pericardium was removed carefully 'v' shaped cut was given in inferior vena cava and the tip of syme's cannula was passed into it. It was tied firmly with inferior vena cava to assure the cannula in place. Immediately the aorta were cutted and carefully heart along with cannula were isolated from the animal, The effect of cardio-selective  adrenergic blocker Metoprolol was added to the biophase in addition to 1 g of adrenaline and the contraction of heart till the 70-80% of inhibition is produced and the difference from normal contraction (inhibition in height of contraction) was recorded. The procedure was repeated by adding -blocker Metoprolol in the dose of 2.5 g, 5 g, 10 g, 20 g, 40 g, 80 g, 160 g and 320 g respectively. Individual findings are recorded on smoked drum cylinder, fixed with resin (colophony) and the recordings are measured. The ID 50 of propranolol and metoprolol was found to be 1.3 and 44 g respectively and interestingly propranolol was more efficient in inhibiting the contraction of adrenaline than metoprolol. These interesting facts can be supported by the finding that  2 receptors predominate in the myocytes of Rana tigrina and this overwhelms the more efficient post-receptor coupling of  1 blockers.