Effects of calcium on a parathyroid hormone-sensitive adenylate cyclase inhibitor

Induction of ornithine decarboxylase activity in isolated chicken osteoblasts by parathyroid hormone: the role of cAMP and calcium

We investigated the role of cAMP and Ca2+ as mediators in parathyroid hormone (PTH)-induced ornithine decarboxylase (ODC) activity in primary cultures of chicken osteoblasts. We present evidence that the induction of ODC activity by PTH is most likely a receptor-mediated process and that cAMP is a mediator. However, using three different approaches we have strong indications that cAMP is not the exclusive mediator of PTH-induced ODC activity. First, when the dose-response curve of PTH-induced ODC activity is compared with that of PTH-stimulated cAMP production, the ED50 for cAMP production is about five times as high as that for the induction of ODC activity. Second, 1 mM 9-(tetrahydro-2-furanyl) adenine (SQ 22.536) almost completely inhibited PTH-stimulated cAMP production whereas there was only a small inhibitory effect on PTH-induced ODC activity. Third, some PTH fragments unable to stimulate cAMP production were still able to induce ODC activity. We therefore propose that apart from cAMP, an additional messenger, most likely Ca2+, must be present. Evidence for this concept are the observations that substances affecting extracellular and intracellular Ca2+ levels (EGTA, A23187, CoCl2, verapamil) or antagonizing calmodulin (Trifluoroperazin, Compound 48/80) also strongly affect PTH-induced ODC activity. These effects could not be explained by a positive interaction of Ca2+ with the hormone-stimulated cAMP system as 2 mM EGTA strongly enhanced PTH-stimulated cAMP production but at the same time completely inhibited PTH-induced ODC activity. A similar dissociation between hormone-induced cAMP production and induction of ODC activity was found with the Ca2+ -ionophore A23187 (10(-7) M) which significantly inhibited PTH-stimulated cAMP production but strongly enhanced PTH-induced ODC activity. Our results suggest that intracellular Ca2+, and possibly calmodulin, in addition to cAMP, are involved in PTH-induced ODC activity in chicken osteoblasts. Most probably Ca2+ is the initial messenger and cAMP acts in a coordinate pattern as a synarchic messenger making the induction of ODC activity by PTH more sensitive to Ca2+. Furthermore, the present findings are in agreement with our concept of the existence of two receptors or two receptor-sites for PTH on osteoblasts. One receptor is coupled to the production of cAMP and is presumably activated when the first two aminoacids of the NH2-terminus of the hormone are present and the other, suggested to be responsible for the increase in intracellular Ca2+, is thought to be activated by a region of the hormone sequence between amino acid 3 and 34.(ABSTRACT TRUNCATED AT 250 WORDS)

Does calmodulin play a role in the regulation of cardiac sarcolemmal adenylate cyclase activity?

The recent suggestion that calmodulin (CaM) could mediate calcium inhibition of cardiac adenylate cyclase (AC) has been reassessed. Using a purified sarcolemmal preparation (SL), the influence of different concentrations of free Ca2+ (obtained using Ca2+-EGTA solutions) was studied on dog heart AC. From 10(-9) M to 10(-3) M Ca2+ reduced basal activity, as well as epinephrine (10(-4) M)- and trypsin (1.0 microgram/mL)-stimulated activities with, in the three cases, an identical IC50 of 10(-8) M. The amount of endogenous CaM in the SL, measured using a radioimmunoassay technique, was found to be 7.5 ng/mg protein. The resulting concentration of CaM in the final AC incubation medium was lower than 50 pM, indicating the lack of a significant role for endogenous CaM in the inhibition observed. The addition of exogenous CaM to the AC assay at a concentration sufficient to stimulate other CaM-dependent systems did not modify the Ca2+ inhibitory curves for basal, epinephrine (10(-4) M)-stimulated, or trypsin (1 microgram/mL)-stimulated activities. These results indicate that CaM does not play a significant role in the Ca2+ inhibition of cardiac AC and that trypsin stimulation of cardiac AC is not mediated through a CaM-dependent process.

Parathyroid hormone: effects of the 3-34 fragment in vivo and vitro

The biologically active fragment ofparathyroid hormone, consisting of residues 1-34, and its in vitro antagonist, fragment 3-34, were administered separately or in combination to chronically thyroparathyroidectomized dogs. These fragments were also studied in vitro with dog renal cortical membranes. Fragment 3-34 inhibited the stimulation of adenylate cyclase by fragment 1-34 in vitro, but had no agonist or antagonistic effects on renal phosphate transport in vivo.

A homologous biological probe for parathyroid hormone in human serum

A method of measuring the biological activity of parathyroid hormone (PTH) in human serum that depends on the activation of its natural target enzyme, human renal cortical adenylate cyclase, is described. Optimal sensitivity ranging in different assays from 14 to 20 pg 1-34 hPTH/ml was achieved in the presence of the GTP-analogue GppNHp (10 mumol/L), 5 mmol/L MgCl2 and 1.25 mmol/L EGTA. Basal and stimulated cAMP production was reproducible within assays (c.v. below 7%, S.E.M., n = 3) and between assays (c.v. 5 to 14%, S.E.M., n = 4). The recovery of 1-34 hPTH added to individual test sera averaged 94%. The specificity of the method was established as follows: 1.) Other tested hormones, at 100 ng/ml, were ineffective; 2.) In the majority of peripheral sera from patients with hyperfunctioning parathyroid glands elevated bio-activity was detected; 3.) The circulating bio-activity fell rapidly after removal of parathyroid adenomata; 4.) Treatment with antisera for hPTH reduced the bio-activity; 5.) A PTH-antagonist inhibited the bio-activity.