CENTRAL RESPIRATORY AND CIRCULATORY EFFECTS OF Gymnodinium breve TOXIN IN ANAESTHETIZED CATS

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Abstract
1 In cats anaesthetized with pentobarbitone, observations were made on respiration, spontaneous and evoked diaphragmatic electromyograms, blood pressure, heart rate, indirectly-induced contractions of the anterior tibialis muscle and nictitating membrane, and electrical excitability of the inspiratory centre in the medulla oblongata. 2 Gymnodinium breve toxin (GBTX) was administered intravenously, intra-arterially to the brain, and intracerebroventricularly. Physiological effects were recorded while alveolar Pco2 was controlled at a constant level except when changes in gas tension were made in order to measure C02-ventilatory responsiveness. 3 Adequate doses of GBTX given intravenously by bolus injection elicited a non-tachyphylactic reflex response triad of apnoea, hypotension and bradycardia mediated by the vagus nerves independently of arterial baroreceptor and chemoreceptor innervation. 4 After vagotomy, additional amounts of GBTX (i.v.) resulted in apneustic breathing, hypertension and tachycardia. The cardiovascular effects were abolished by ganglionic blockade with hexamethonium. 5 Smaller doses of GBTX were required intra-arterially and intracerebroventricularly than by the intravenous route of injection to produce respiratory irregularity and cardiovascular hyperactivity. 6 Evoked motor responses, electrical excitability of the medulla oblongata and CO2-ventilatory responsiveness were largely spared even though GBTX caused marked disturbances in respiratory rhythmicity and cardiovascular functions. 7 It is concluded that GBTX acts reflexly on vagally innervated receptors to evoke a Bezold-Jarisch effect but that the toxin further acts centrally to cause irregular breathholding and hypertension with tachycardia, leading ultimately to respiratory and circulatory failure.