Cholinergic and adrenergic tons in the control of heart rate in teleosts. How should they be calculated?

Cholinergic and adrenergic tones were calculated for three different teleost fish species: Gadus morhua, Labrus bergylta, and Sparus aurata using atropine as a muscarinic receptor antagonist and either sotalol or propranolol as β-adrenoceptor antagonists. Depending on the order of administration of atropine and the two β-adrenoceptor antagonists, it was observed that propranolol but not sotalol enhanced cholinergic tone. Thus, if propranolol is used to determined autonomic cardiac influences, it has to be injected after atropine and not before. Differences in intrinsic heart rate were observed between treatments in two of the three species studied, suggesting the activity of a non-cholinergic non-adrenergic mechanism in heart rate control in fish. Different models to calculate cholinergic and adrenergic tones are discussed. The additive model described by other authors is appropriate provided that no interaction exists between cholinergic and adrenergic influences. We demonstrate no interaction in the species studied in this experiment. Finally, a modification of the additive model that uses R-R interval instead of heart rate in the computation is proposed. This is justified with a computer simulation in terms of the linearity of the response given the reciprocal relationship between R-R interval and heart rate.