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Estimulos de alarme induzem inibicao temporaria dos batimentos cardiacos em muitos animais. Como essa proprieade, denominada parada (arrest) cardiaca reversivel nao vem sendo estudada sistematicamente em peixes, os presente trabalho investigou a sua ocorrencia em peixes antarticos que evoluiram independentemente de outros grupos de teleosteos. Alteracoes do ritmo cardiaco em resposta a diferentes estimulos foram monitoradas por registro eletrocardiografico em 15 especimes de Notothenia neglecta (400-700 g) mantidas em uma câmara experimental com agua do mar aerada e temperatura controlada. Vibracoes repentinas da câmara, a aproximacao do observador e outros estimulos de alerta provocavam no animal paradas cardiacas algumas vezes por um ou mais batimentos, mas a resposta mais comum era somente de uma parada curta do batimento cardiaco apos estimulo. Aumento da temperatura ate 11oC eleva o ritmo (rate) cardiaco sem abolir a resposta contudo. Hipoxia moderada facilita a parada cardiaca. Inibicao cardiaca pronunciada com algum batimento isolado foi observada quando os animais eram manuseados ou quando a agua da câmara era removida. Administracao de atropina (1 mg/kg) bloqueia a parada cardiaca induzida por tais procedimentos. Conclui-se, entao, que a parada cardiaca reversivel e provavelmente uma propriedade comum de muitos peixes que se desenvolveu antes da sua adaptacao ao ambiente antartico. Abstract Startling stimuli induce temporary inhibition of heart beats in many animals. Since such property, named reversible cardiac arrest, has not been systematically studied in fishes, the present work investigated its occurrence in an Antarctic fish that evolved independently from other teleost groups. Heart rate changes in response to different stimuli were monitored by conventional electrocardiographic recording in 15 Notothenia neglecta (400-700 grams), maintained in an experimental chamber with aerated sea water and controlled temperature. Sudden vibration of the chamber, observer approximation and other alerting stimuli evoked cardiac arrest sometimes for one or more beats, but the most common response was only a short delay of the beat after the stimulus. Increase in temperature up to 11oC raised heart rate but did not abolish the response. Moderate hypoxia facilitated cardiac arrest. Pronounced cardiac inhibition with some isolated beats was observed when the animals were handled or when the water of the chamber was removed. Atropine (1mg/kg) administration blocked the cardiac arrest induced by such procedures. It is concluded that reversible cardiac arrest is probably a common property of many fishes and developed before the adaptation to the Antarctic environment.

REVERSIBLE CARDIAC ARREST INDUCED BY STARTLING STIMULI IN THE ANTARCTIC FISH Notothenia neglecta