No abstract

No abstract

In several water-breathing fish species, β-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 β-agonist, isoprenaline. To identify the receptor type involved, a bolus of either a selective β 1 -antagonist (atenolol) or a selective β 2 -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 β 1 -and β 2 -antagonist led to higher glucose concentrations than isoprenaline alone. This could indicate the presence of a stimulatory β-adrenoceptor different from β 1 and β 2 -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 β 1 -antagonist had no effect, the β 2 -antagonist reduced the decrease in FFA concentration, indicating the involvement of β 2 -adrenoceptors. It is concluded that the air-breathing African catfish reflects water-breathing fish in the adrenergic control of plasma FFA and glucose levels.

African catfish were cannulated in the dorsal aorta to study diurnal changes in blood metabolites. Cannulation of the branchial artery was tested but proved to be less successful. A clear diel fluctuation in the two major blood metabolites, free fatty acids (FFA) and glucose, was observed. Compared to the initial value at 8.30 a.m., the plasma FFA levels dropped by ca. 50% within 2 hours, after which the FFA concentration stayed relatively constant. Minimum values of 0.26±0.04 mM were reached at 12.30. The FFA concentration recovered to the initial value within the following 3 hours. The fluctuation in plasma glucose levels showed a comparable course but there was a phase-shift by 2 hours. The most astonishing finding of our study was the almost complete absence of glucose in the plasma of African catfish (0.05 ± 0.03 mM), a never reported phenomenon for any fish species. This study demonstrates the relatively low level of control of plasma glucose levels as compared to plasma FFA levels in African catfish.