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Background  In the anoxia-tolerant crucian carp ( Carassius carassius ) cardiac activity varies according to the seasons. To clarify the role of autonomic nervous control in modulation of cardiac activity, responses of atrial contraction and heart rate (HR) to carbacholine (CCh) and isoprenaline (Iso) were determined in fish acclimatized to winter (4°C, cold-acclimated, CA) and summer (18°C, warm-acclimated, WA) temperatures.    Results  Inhibitory action of CCh was much stronger on atrial contractility than HR. CCh reduced force of atrial contraction at an order of magnitude lower concentrations (EC 50  2.75-3.5·10 -8  M) in comparison to its depressive effect on HR (EC 50  1.23-2.02·10 -7  M) (P < 0.05) without differences between winter and summer acclimatized fish. Inhibition of nitric oxide synthase with 100 μM L-NMMA did not change the response of the sinoatrial tissue to CCh. Reduction of atrial force was associated with a strong shortening of action potential (AP) duration to ~50% (48 ± 10 and 50 ± 6% for CA and WA fish, respectively) and 11% (11 ± 3 and 11 ± 2% for CA and WA fish, respectively) of the control value at 3·10 -8  M and 10 -7  M CCh, respectively (P < 0.05). In atrial myocytes, CCh induced an inwardly rectifying K +  current, I K,CCh , with an EC 50  value of 3-4.5·10 -7  M and inhibited Ca 2+  current (I Ca ) by 28 ± 8% and 51 ± 6% at 10 -7  M and 10 -6  M, respectively. These currents can explain the shortening of AP. Iso did not elicit any responses in crucian carp sinoatrial preparations nor did it have any effect on atrial I Ca , probably due to the saturation of the β-adrenergic cascade in the basal state.    Conclusion  In the crucian carp, HR and force of atrial contraction show cardio-depressive responses to the cholinergic agonist, but do not have any responses to the β-adrenergic agonist. The scope of inhibitory regulation by CCh is increased by the high basal tone of the adenylate cyclase-cAMP cascade. Higher concentrations of CCh were required to induce I K,CCh  and inhibit I Ca  than was needed for CCh's negative inotropic effect on atrial muscle suggesting that neither I K,CCh  nor I Ca  alone can mediate CCh's actions but they might synergistically reduce AP duration and atrial force production. Autonomic responses were similar in CA winter fish and WA summer fish indicating that cardiac sensitivity to external modulation by the autonomic nervous system is not involved in seasonal acclimatization of the crucian carp heart to cold and anoxic winter conditions.

Sinoatrial tissue of crucian carp heart has only negative contractile responses to autonomic agonists