Effects of acute anoxia on heart function in crucian carp: importance of cholinergic and purinergic control.

The objective of this study was to characterize the effects of acute anoxia on contractile and electrical activity in the heart of an anoxia-tolerant fish species, the crucian carp ( Carassius carassius L.). Responses of atrial and ventricular tissue or isolated cells to NaCN, adenosine, and carbachol were determined to examine the effects of anoxia on cardiac performance and to clarify the possible role of local purinergic modulation and parasympathetic nervous control in the function of the anoxic fish heart. The contractility of the crucian carp heart is strongly decreased by acute anoxia. A rapid reduction in cardiac contractility is attained by reflex bradycardia and suppression of atrial contractility. These responses are mediated by muscarinic cholinergic receptors through the opening of inwardly rectifying potassium channels and are likely to protect the cardiac muscle from hypoxic/anoxic damage. The depletion of tissue oxygen content also directly depresses heart rate and cardiac force. Ultimately, an increase in cytosolic Ca2+ concentration occurs that activates sarcolemmal Ca2+extrusion through the Na+-Ca2+-exchange and generates an inward exchange current with consequent depolarization of the resting membrane potential and possible cell death. At physiological concentration, the effects of adenosine on contractile and electrical activity were relatively weak, suggesting that the purinergic system is not involved in the acute anoxia response of the crucian carp heart.