PARATHYROID-HORMONE (PTH) PTH-RELATED PEPTIDE RECEPTOR DENSITY MODULATES ACTIVATION OF PHOSPHOLIPASE-C AND PHOSPHATE-TRANSPORT BY PTH IN LLC-PK1 CELLS/

We showed previously that a single species of cloned PTH/PTH-related p eptide (PTHrP) receptors, when stably expressed in LLC-PK1 kidney cell s, couples to multiple second messenger signals and biological respons es. To address the linkages of individual messenger signals to specifi c biological responses in these cells, we examined the relations among PTH/PTHrP receptor expression, PTH-activated phospholipase C (PLC) an d adenylyl cyclase, and PTH-regulated phosphate transport in LLC-PK1 c ells that stably express cloned rat PTH/PTHrP receptors. Among 18 such subclones, PTH stimulation of intracellular cAMP accumulation was nea rly equivalent, despite differences in receptor density ranging from 2 0,000-400,000 sites/cell. In contrast, activation of PLC by PTH was di rectly and continuously dependent upon receptor density. PTH-stimulate d phosphate uptake also was strongly dependent upon receptor expressio n, correlated well with PLC activity, was mimicked by active phorbol e sters but not by cAMP analogs or forskolin, and was strikingly inhibit ed by the protein kinase C inhibitor, staurosporine. The peptide analo g [Arg(2)]human PTH-(1-34), which significantly stimulated cAMP accumu lation but failed to activate PLC, also did not increase phosphate upt ake. We conclude that in LLC-PK1 cells, PTH-modulated PLC activation, unlike adenylyl cyclase activation, is strongly dependent upon PTH/PTH rP receptor density. This feature is reflected in the analogous relati on between receptor density and PTH regulation of phosphate uptake, wh ich appears to be mediated via a PKC-dependent pathway in these transf ected cells. The results suggest that regulation of PTH/PTHrP receptor expression on target cells may provide a mechanism for altering the c haracter as well as the magnitude of the signaling response to the hor mone.