The Parathyroid Hormone Receptor-Adenylate Cyclase System in Chicken Kidney

A B S T R A C T The designing of parathyroid hormone (PTH)-renal dose-response studies in human beings is complicated by the possibility of rapid homologous receptor down-regulation, a phenomenon that is clearly shown to occur in vitro. Large amounts of PTH given to human subjects as serial injections or prolonged infusions cause decreased urinary 3',5'-cyclic adenosine monophosphate (cAMP) responses to subsequent PTH doses, but it is uncertain whether lower doses given over shorter periods similarly cause renal tachyphylaxis to PTH action. Thus, in seven water-loaded adults, we infused in ascending order 10, 30, 75, 150, and 300 U of synthetic bovine PTH fragment (bPTH 1-34) per 70 kg body wt over 15 min on widely separated days ("separate day administration"). On another day, each subject received all five 15-min doses in ascending order at 75-min intervals ("single day administration"). Urine collection intervals were control, 0-30 min (including the PTH infusion), and 30-60 min. Peak nephrogenous cAMP (NcAMP, nmol/100 ml glomerular filtrate) response was linearly related to the dose of PTH (separate day study, r = 0.94, P < 0.001; single day study, r = 0.88, P < 0.001). However, the slope of NcAMP responses plotted against PTH dose for the single day study was only 36% of that derived from separate day administration of the same PTH doses (P < 0.001). After only 40 U (10 + 30) of bPTH 1-34/70 kg, the NcAMP response to 75 U was reduced 44%, and the effect of 300 U/70 kg, when given as the last of the sequential single day infusions, was 64% less than the response to 300 U of bPTH 1-34 given alone (P < 0.001). The phosphaturic response (fractional excretion of phosphorus, FEP [percent]) was also linearly related to bPTH 1-34 dose, but combined administration of the PTH infusions on one day increased FEP at each dose identically with the effects of separate day administration. A transient, dose-related, early hypercalciuric response to bPTH 1-34 also occurred, and was of equal magnitude in both protocols. These studies demonstrate that significant blunting of the NcAMP response to bPTH 1-34 occurs rapidly and follows brief exposure to relatively low doses of hormone. In contrast, there is no effect of recent PTH administration on the phosphaturic and early hypercalciuric actions of bPTH 1-34. This seeming dissociation of PTH effects makes unclear the physiologic importance of PTH-induced cAMP tachyphylaxis in the regulation of final PTH actions. In any case, studies of NcAMP responses in which the occurrence of tachyphylaxis would be undesirable should be designed to avoid prolonged or closely spaced administrations of the hormone. INTRODUCTION The acute administration of exogenous parathyroid hormone (PTH)' causes increased urinary excretion of

Section: Resultsmentioning

confidence: 99%

Rapid development of renal resistance to low doses of synthetic bovine parathyroid hormone fragment 1-34. Dissociation of urinary cyclic adenosine monophosphate, phosphaturic, and calciuric responses.

Law¹,

Heath²

1983

“…Highly purified renal cortical plasma membranes were prepared from kidneys immediately after removal by a modification (21) of the method of Fitzpatrick et al (22). Membranes were stored in aliquots at -80°C.…”

Section: Methodsmentioning

confidence: 99%

“…Incubations were carried out for 60 min at 30°C. Bound '25I-bPTH was separated by filtration through 0.2-sgm cellulose acetate filters and analyzed as previously described (21). Binding in the presence of 700 nM bPTH (1-34) was considered nonspecific.…”

Section: Methodsmentioning

confidence: 99%

Canine renal receptors for parathyroid hormone. Down-regulation in vivo by exogenous parathyroid hormone.

Mahoney¹,

Nissenson²

1983

A B S T R A C T Chronic elevation of circulating parathyroid hormone (PTH) is associated with decreased target cell responsiveness to PTH. To study the subcellular mechanism of this phenomenon we evaluated PTH receptors and adenylate cyclase activity in renal cortical membranes prepared before and after infusion of bovine parathyroid gland extract (PTE) into thyroparathyroidectomized dogs. PTE infusion resulted in a 53% decrease in the number of high-affinity receptors (P < 0.01) associated with a 66% decrease in PTH-stimulated adenylate cyclase (P < 0.01) relative to paired base-line values. Both the equilibrium constant of dissociation (KD) for PTH binding and the concentration of PTH that caused half-maximal stimulation of adenylate cyclase were in the range of 1 to 4 nM, and were unaffected by the PTE infusion. Responsiveness of the renal adenylate cyclase to sodium fluoride was 88% of base-line values. Infusion of the PTE vehicle alone did not affect PTH receptor number or blunt the adenylate cyclase response to PTH.Pretreatment of the membranes made after PTE infusion with guanosine triphosphate (GTP), which is known to produce dissociation of receptor-bound PTH, failed to restore either receptor number or PTH-stimulated adenylate cyclase. This finding was not due to a lack of efficacy of the GTP pretreatment, because identical GTP pretreatment restored PTH incubation with PTH in vitro. Thus, simple residual occupancy of PTH receptors by the infused hormone did not appear to account for the observed receptor loss.The results of this study suggest that target cell resistance to PTH in patients with hyperparathyroidism might occur, at least in part, due to down-regulation of PTH receptors by circulating hormone.

“…Synthetic (syn) bPTH was added at the indicated concentrations. The binding of the radioligand to kidney membranes is specific and saturable and correlates with biologic activity in the adenylate cyclase assay (14,19,20). The incubations were performed at 16°C for 60 min.…”

Section: Methodsmentioning

confidence: 99%

Effects of dietary-induced hyperparathyroidism on the parathyroid hormone-receptor-adenylate cyclase system of canine kidney. Evidence for postreceptor mechanism of desensitization.

Tamayo¹,

Bellorín-Font²,

Martin³

1983

A B S T R A C T The present studies were designed to examine the consequences of chronic mild elevations of endogenous parathyroid hormone (PTH) in vivo on the PTH receptor-adenylate cyclase system of canine kidney cortex. Hyperparathyroidism was induced in normal dogs by feeding a diet low in calcium, high in phosphorus to the animals for a period of 6-9 wk. This maneuver resulted in a two to threefold increase in the plasma levels of carboxy-terminal immunoreactive PTH. This degree of hyperparathyroidism is similar to that seen in patients with hyperparathyroidism and normal renal function. After 6-9 wk on the diet the animals were killed and basolateral renal cortical membranes prepared for the study of the PTH receptor-adenylate cyclase system in vitro. The dietary hyperparathyroidism resulted in desensitization of the PTH-responsive adenylate cyclase (Vmax 3,648±654 pmol cyclic (c)AMP/mg protein per 30 min in hyperparathyroid animals vs. 5,303±348 in normal controls). The Ka.t (concentration of PTH required for half-maximal enzyme activation) was unchanged. However, PTH receptor binding '251[Nle , Nle'8, Tyr34] bPTH (1-34) NH2 as radioligand) was not different in the two groups of animals. Thus, dietary hyperparathyroidism resulted in an uncoupling of the PTH receptor-adenylate cyclase system. This defect was not corrected