CHARACTERIZATION OF NA+, K+ ATPASE IN ERYTHROCYTES OF DAIRY-COWS - EFFECT OF DIET AND GENETIC SELECTION FOR MILK SOLIDS

The number and activity of Na+, K+ ATPase units were assessed with H-3-ouabain binding and Rb-86(+) uptake methods, respectively, in erythrocytes from second-parity Holstein-Friesian cows at weeks 10 and 24 of lactation. The cows were either from S or C genetic lines for yield of milk fat and protein and were offered diets of either H or L forage content. The Scatchard method of analysis indicated that H-3-ouabain binding was compatible with a single class of receptors of high affinity. S-line cows had higher milk yields (P < 0.001) than C-line animals at both sampling points. Those animals fed the low-forage feed had higher levels of dry matter intake (P < 0.001) and milk yields (P < 0.001) at weeks 10 and 24 than contemporaries fed the high-forage feed. Main effects of genetic line and diet were not significant for dissociation-rate constants (K-d) and the number and activity of Na+, K+ ATPase units. The number of enzyme units was higher at week 24 than at week 10 (P < 0.1), and between-cow variation was significant for the number of enzyme units (P < 0.02). A significant genetic line x diet interaction for enzyme activity at weeks 10 and 24 was detected (P < 0.10). This was particularly pertinent at week 10, when activity was greatest for groups CH and SL and least for groups CL and SH. The reduction in ouabain-sensitive Rb-86(+) uptake for groups CL and SH may reflect an imbalance between genotype and nutrient provision. Positive relationships were found between the number and activity of Na+, K+ ATPase units and the milk energy output (P < 0.1 and P < 0.01, respectively). This may reflect the increased demand in Na+, K+ pumping that is required for enhanced milk productivity and suggests that the activity of erythrocyte Na+, K+ ATPase reflects the activity of this enzyme in other tissues during lactation.