The sensitivity of frog hearts to acetylcholine and other neurohumoral transmitter and allied substances and the comparative toxicity of modern plastic and silicone connecting tubing tested on frog hearts

Abstract

The thesis is divided into two parts dealing with separate topics: Part I - The Sensitivity of Frog Hearts to Acetylcholine and Other Neurohumoral Transmitter and Allied Substances. Part II - The Toxicity of Modern Plastic and Silicone Connecting Tubing, Tested on Frog Hearts. The aim of the present work was to investigate the threshold sensitivity (minimum effective concentrations) of frog hearts to some well documented substances of physiological importance under critically controlled experimental conditions. Acetylcholine, adrenaline, noradrenaline, 5-hydroxytryptamine and nicotine were selected for this purpose. Very sensitive electronic equipment specially designed and adapted to suit the frog heart was developed over a period of several years. The equipment permits a well controlled perfusion of frog heart and records the perfusion pressure (venous pressure) heart rate, arterial pressures (systolic, diastolic and pulse pressure) and cardiac output simultaneously and continuously. The electronic part of the equipment essentially consists of a specially designed 4 channel direct ink writing recorder. Photographs of different parts of equipment and operational details are given in the section on Methods. The perfusion of frog heart was conducted by introducing a cannula into the inferior vena cava without opening the pericardium. The perfusate coming out of the two aortic cannulae passed through an 'arterial system' consisting of elastic connecting tubing and artificial resistance, and finally escaped out in the form of drops which were counted and integrated electronically. Since the constancy of venous pressure was a crucial point in providing a stable record and also in preventing the occurrence of undesirable changes in the different recorded parameters during a critical test perfusion, special care was exercised in designing and developing the venous pressure recorder. The venous pressure recorder and the carriages for the different sources of perfusion have been so developed that the venous pressure can be kept constant at any desired value throughout the period of experiment lasting from 1 to 3 days. At the same time unavoidable minor changes in the venous pressure (of course quite insufficient to influence the stability of record) of the order of 0.05 cm of even less, were recorded and displayed very prominently. A venous pressure monitoring meter constantly guided the operator. Special methods of precaution and control were introduced at several stages of the experiment so that the effect of test solutions were well controlled and it was clearly shown on the record that there could be no possibility of unrecognised changes in the record due to artefacts - electronic or otherwise. The distilled water used for preparing solutions, the ionic composition of fluids, the type of glass containers, the connecting tubing, the minor variations in temperature from standing at different places), minor variations in the pH of the solutions and bacterial, chemical or organic contamination of glassware and tubing, all were liable to influence the behaviour of the preparation. With conventional crude methods the influence of such factors would never be detected. Most of these variables were easily controlled once their importance was realised, though a considerable time was devoted to analysing and controlling them. (Abstract shortened by ProQuest.)