The effects of pH on chicken liver xanthine dehydrogenase

Replete ( 0 ) and vitamin D-deficient ( A ) cells were incubated at pH 7.4 and 37°C for thc indicated time intervals. Po is the P, concentration at the start of the incubation; P, is the measured concentration at t min. The results are the means ~ s . E . for three experiments performed in duplicate. medium containing EGTA ( I mM) but with no added C a 2 + . This efflux into Ca2+-free medium was compared with the rate observed with cells suspended in the usual medium that contains CaCI, (1.2 mM) but no EGTA. No effect on P, of a Ca2+-free environment could be detected. To test the effect of parathyrin on P, efflux, normal cells were incubated in medium containing parathyrin ( 1 unit/ml) or parathyrin ( 1 unit/ml) plus 0.1 mM-theophyhe and the P, efflux rate compared with that from cells incubated in the absence of the hormone or phosphodiesterase inhibitor. The cells were exposed to parathyrin and/or theophylline for I min before first sampling the mixture for P, determination. Efflux was followed for up to 60 min by sampling at appropriate time intervals. The parathyrin, or parathyrin plus theophylline, was present throughout the whole of the 60 min incubation period. Parathyrin alone had no significant effect on P, efflux, but in combination with theophylline there was a 2&30'/0 inhibition of the efflux rate. Thus 3'3'cyclic AMP may have some influence on the activity of the basolateral P, transporter. Further experiments are planned in order to establish the mechanism by which parathyroid hormone plus theophylline act to decrease P, efflux.