Parathyroid hormone (PTH) leads to the inhibition of Na-P-i cotransport act
ivity and to the downregulation of the number of type II Na-P-i cotransport
ers in proximal tubules, as well as in opossum kidney (OK) cells. PTH is kn
own also to lead to an activation of adenylate cyclase and phospholipase C
in proximal tubular preparations, as well as in OK cells. In the present st
udy, we investigated the involvement of these two regulatory pathways in OK
cells in the PTH-dependent downregulation of the number of type II Na-P-i
cotransporters. We have addressed this issue by using pharmacological activ
ators of protein kinase A (PKA) and protein kinase C (PKC), i.e., 8-bromo-c
AMP (8-BrcAMP) and beta-12-O-tetradecanoylphorbol 13-acetate (beta-TPA), re
spectively, as well as by the use of synthetic peptide fragments of PTH tha
t activate adenylate cyclase and/or phospholipase C, i.e., PTH-(1-34) and P
TH-(3-34), respectively. Our results show that PTH signal transduction via
cAMP-dependent, as well as cAMP-independent, pathways leads to a membrane r
etrieval and degradation of type II Na-P-i cotransporters and, thereby, to
the inhibition of Na-P-i cotransport activity. Thereby, the cAMP-independen
t regulatory pathway leads only to partial effects (similar to 50%).