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

The number and activity of Na+, K+ ATPase units were assessed with H-3 -ouabain binding and Rb-86(+) uptake methods, respectively, in erythro cytes 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 c ontent. The Scatchard method of analysis indicated that H-3-ouabain bi nding 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 a t both sampling points. Those animals fed the low-forage feed had high er 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 dissociatio n-rate constants (K-d) and the number and activity of Na+, K+ ATPase u nits. 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 o f enzyme units (P < 0.02). A significant genetic line x diet interacti on for enzyme activity at weeks 10 and 24 was detected (P < 0.10). Thi s was particularly pertinent at week 10, when activity was greatest fo r groups CH and SL and least for groups CL and SH. The reduction in ou abain-sensitive Rb-86(+) uptake for groups CL and SH may reflect an im balance between genotype and nutrient provision. Positive relationship s were found between the number and activity of Na+, K+ ATPase units a nd the milk energy output (P < 0.1 and P < 0.01, respectively). This m ay reflect the increased demand in Na+, K+ pumping that is required fo r enhanced milk productivity and suggests that the activity of erythro cyte Na+, K+ ATPase reflects the activity of this enzyme in other tiss ues during lactation.