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

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 P co 2 was controlled at a constant level except when changes in gas tension were made in order to measure C0 2 ‐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 CO 2 ‐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.