CHARACTERIZATION OF CALCIUM-TRANSPORT IN THE LUMINAL AND THE BASOLATERAL MEMBRANE FROM KIDNEY CORTEX OF HYPERCALCEMIC RATS BEARING THE WALKER-256 CARCINOSARCOMA

The Walker 256 (W256) carcinosarcoma did not significantly modify calc ium uptake by brush border membrane (BBM) vesicles in the kidney of th e host rat, compared with that in membranes isolated from control anim als. However, it showed a tendency to increase at near equilibrium in W256 tumor-host rats, associated with a decreased BBM protein content. ATP-dependent calcium influx by basolateral membrane (BLM) vesicles f rom W256 tumor-bearing rats was also increased compared with that in c ontrol BLM. This stimulation was due to a decreased K(m) for calcium. Passive calcium permeability or the Na+/Ca2+ exchanger were unchanged in BLM from W256 tumor-host rats compared with control BLM. Pre-stimul ation of control rat cortical tubules with either 10(-7) M parathyroid hormone-related protein (PTHrP) (1-34) or 10(-4) M N6,2'-O-dibutiryl cyclic AMP before BLM isolation, did not modify the ATP-dependent calc ium uptake by BLM vesicles compared with control membranes. However, o ur results do not rule out that the stimulated ATP-dependent calcium i nflux in BLM from W256 tumor-host rats could be mediated by the intera ction among PTHrP and other humoral factors. Our findings suggest a po ssible mechanism for the increased renal calcium reabsorption in this animal model for humoral hypercalcemia of malignancy.