The parathyroid hormone 2 (PTH2) receptor

The human PTH2 receptor binds and is activated at high potency by PTH and by the recently discovered peptide tuberoinfundibular peptide of 39 residues (TIP39). Rat and zebrafish PTH2 receptors are more weakly activated by PTH, suggesting that TIP39 may be the natural ligand for the PTH2 receptor. Unlike the PTH1 receptor, the PTH2 receptor interacts extremely weakly with parathyroid hormone-related peptide (PTHrP). The PTH2 receptor is strongly coupled to stimulation of cAMP accumulation, and more weakly, in a cell-specific manner to increases in intracellular calcium concentration. A variety of evidence supports the general model of receptor amino terminal sequences binding ligand carboxyl terminal sequences with high affinity, and ligand amino terminal sequences activating the receptor through interaction with the "juxtamembrane" portion of the receptor. The receptor is present at greatest levels in the nervous system. It is expressed in scattered cells in the cerebral cortex and basal ganglia and at relatively high abundance in the septum, midline thalamic nuclei, several hypothalamic nuclei, and the dorsal horn of the spinal cord. Peripherally, expression in pancreatic islet somatostatin cells is most dramatic. Functional hypotheses based on the receptor's distribution are being tested. Recent data support involvement in hypothalamic releasing-factor secretion and pain.

Regulation of human D(1), d(2(long)), d(2(short)), D(3) and D(4) dopamine receptors by amiloride and amiloride analogues

1. The modulatory effects of the allosteric effectors methylisobutylamiloride (MIA), benzamil and amiloride have been examined at human D(1), D(2), D(3) and D(4) dopamine receptors. The subtype selectivity and the mechanism of action of this allosteric regulation was examined. 2. In radioligand dissociation experiments each modulator accelerated dissociation from all four receptor subtypes indicating allosteric regulation. MIA displayed selectivity for the D(3) subtype for acceleration of radioligand dissociation. 3. In equilibrium binding (pseudo-competition) experiments the three compounds inhibited radioligand binding at the four receptor subtypes. Inhibition curves for D(1), D(2(short)), D(2(long)) and D(3) receptors were described by Hill coefficients exceeding unity and data were fitted best by a model that assumes binding of modulator to both the primary and allosteric binding sites of the receptor (the allosteric/competitive model). 4. At the D(4) subtype, Hill coefficients of unity described the binding data for amiloride and benzamil, consistent with competitive inhibition. The Hill coefficient for MIA at the D(4) subtype was less than unity and data could be fitted well by the allosteric/competitive model, but it was not possible to define unambiguously the modulatory mechanism. For this effect a better definition of the mechanism could be obtained by simultaneous analysis of data obtained in the presence of a range of concentrations of a purely competitive ligand. 5. MIA reduced the potency with which dopamine stimulated [(35)S]-GTPgammaS binding at the D(2) receptor. The effects of MIA could be described by the allosteric/competitive model with effects of MIA to inhibit the binding of dopamine but not its ability to induce a response.