Beta-adrenergic control of plasma glucose and free fatty acid levels in the air-breathing African catfishClarias gariepinusBurchell 1822

In several water-breathing fish species, beta-adrenergic receptor stimulation by noradrenaline leads to a decrease in plasma free fatty acid (FFA) levels, as opposed to an increase in air-breathing mammals. We hypothesised that this change in adrenergic control is related to the mode of breathing. Therefore, cannulated air-breathing African catfish were infused for 90 min with noradrenaline or with the nonselective beta-agonist, isoprenaline. To identify the receptor type involved, a bolus of either a selective beta1-antagonist (atenolol) or a selective beta2-antagonist (ICI 118,551) was injected 15 min prior to the isoprenaline infusion. Both noradrenaline and isoprenaline led to an expected rise in glucose concentration. Isoprenaline combined with both the beta1- and beta2-antagonist led to higher glucose concentrations than isoprenaline alone. This could indicate the presence of a stimulatory beta-adrenoceptor different from beta1 and beta2-adrenoceptors; these two receptors thus seemed to mediate a reduction in plasma glucose concentration. Both noradrenaline and isoprenaline led to a significant decrease in FFA concentration. Whereas the beta1-antagonist had no effect, the beta2-antagonist reduced the decrease in FFA concentration, indicating the involvement of beta2-adrenoceptors. It is concluded that the air-breathing African catfish reflects water-breathing fish in the adrenergic control of plasma FFA and glucose levels.