A STUDY OF THE INFLUENCE OF AMMONIA, TWE AMMONIUM, AND PH GRADIENTS ON THE ACCUMULATION OF AMMONIA IN THE BLOOD OF CHANNEL CATFISH, ICTALURUS PUNCTATUS

JAMES PHILLIP SWIGERT, Purdue University

Abstract

The dynamics of net ammonia flux across the catfish gill membrane was studied by direct sampling of aortic blood in cannulated fish. The permeability of the catfish gill membranes to NH(,3) and NH(,4)('+) was compared to that of a dialysis membrane beg. Plasma within dialysis bags exhibited steady-state concentrations of total ammonia which approximated the total ammonia in the exposure solutions. Whole fish exposures to NH(,4)('+) gradients as high as 53 times the normal plasma concentration for 2 h had no measurable effect on the plasma total ammonia concentrations. Rather, the steady-state concentration of total ammonia varied depending on the pH and total ammonia concentration of the exposure solution. However, a positive linear relationship (r('2) = 0.97) was found between exposure un-ionized ammonia and plasma un-ionized ammonia that was independent of pH. Concentrations of un-ionized ammonia in plasma were 50% of expected. The uptake data were used to calculate rate constants for uptake (Ku), elimination (Kd), and bioconcentration factors (BCF = Ku/Kd). The Ku for pooled replicates varied from 0.09 to 0.38 min('-1), while elimination rate constants obtained from the uptake model were 0.06 to 0.24 min('-1). Separate elimination experiments demonstrated that ammonia clearance from loaded catfish followed a biphasic pattern with 'slow' and 'fast' rate constants of 0.05 min('-1) and 0.09 min('-1), respectively. Bioconcentration factors varied directly with increasing pH. When exposure pH was greater than blood pH (8.4 vs 7.8) the BCF ranged from 1.4 to 3.4. When pH in the water was less than that of blood, either no uptake was observed (at pH 6.4) or BDFs ranged from 0.6 to 0.8 for the pH 7.7 experiments. Because of the relative impermeability of the catfish gill to NH(,4)('+) ions, the BCF can be used to predict plasma total ammonia if the exposure total ammonia and pH are known. Although an ion-trapping theory described the steady-state distribution of total ammonia in plasma, the observed concentrations actually fell short of what would be predicted. The most likely explanation for the discrepancy appears to be a pH shift in the water surrounding the gill caused by excretion of acids.

Degree

Ph.D.

Subject Area

Zoology

COinS