Bovine parathyroid cells

Brown, EM, Fuleihan, Ge-H, Chen, CJ and Kifor, O, 1990, A comparison of the effects of divalent and trivalent cations on parathyroid hormone release, 3, 5 -cyclic-adenosine monophosphate accumulation, and the levels of inositol phosphates in bovine parathyroid cells, Endocrinology 127 1064-1071... 

Nemeth, E and Scarpa, A, 1987, Rapid mobilization of cellular Ca2+ in bovine parathyroid cells by external divalent cations, J Biol Chem 202 5188-5196 Nemeth, EF, Fox, J, Delmar, EG, Steffey, ME, Lambert, LD, Conklin, RL, Bhatnagar, PK and Gowen, M, 1998a, Stimulation of parathyroid hormone secretion by a small molecule antagonist of the calcium receptor., J Bone Miner Res 23 S156 (absract 1030)... 

D-l Dopamine Receptor-Mediated Activation oi Adenylate Cyclase, cAMP Accumulation, and PTH Release in Dispersed Bovine Parathyroid Cells... 

The evidence supporting the existence of a specific category of dopamine receptor on the parenchymal cells of the bovine parathyroid gland and the possible biochemical mechanisms by which dopamine stimulates the release of parathyroid hormone are reviewed. The dopamine receptor on the bovine parathyroid cell is compared to other dopamine receptors. 

The cellular and molecular events involved in the dopamine-stimulated release of PTH can be clarified in experiments utilizing bovine parathyroid cells dispersed with collagenase and DNase (ft). This dispersion procedure yields parenchymal cells with only a slight contamination by red blood cells. The parenchymal cells exclude trypan blue and appear normal by light and electron microscopy (ft). These cells release PTH in a linear fashion for several hours the release is inhibited by calcium and stimulated by dopamine and beta-adrenergic agonists at concentrations comparable to those used to elicit physiological responses in vivo (ft,ft). 

Dopamine Enhances PTH Secretion in Dispersed Bovine Parathyroid Cells... 

Dopamine causes a 20 to 30-fold increase in the content of cAMP in dispersed bovine parathyroid cells (Figure 4) (ft). 

Figure 1. Stimulation of PTH release from dispersed bovine parathyroid cells by dopamine. Cells were incubated with (M) or without ( j 1 /tM dopamine, and PTH release was determined by radioimmunoassay.

Figure 4. Stimulation of cAMP accumulation in dispersed bovine parathyroid cells by varying concentrations of dopamine (O), 6,7-ADTN (Cl), SKF 38393 (A), or apomorphine (9). (Reproduced with permission from Ref. 10. Copyright 1980, American Society for Pharmacology and Experimental Therapeutics.)...

When tested on osmotically-lysed bovine parathyroid cells, dopamine enhances the activity of adenylate cyclase, the enzyme converting ATP to cAKP (111. In comparison with its effect on cAMP accumulation, the effect of dopamine on adenylate cyclase activity is relatively modest, only a 2-fola increase in enzyme activity (Figure 7). Guanosine 5 -triphosphate (GTP) increases the stimulatory effect of dopamine in the presence of GTP, there is 3 to i<-fold stimulation of enzyme activity (11). 

Figure 5. Stimulation of intracellular (0) and extracellular (O) cAMP by 10 /tM dopamine in dispersed bovine parathyroid cells. cAMP was determined by radioimmunoassay.

Affinity of drugs for the dopamine receptor in bovine parathyroid cells determined in experiments measuring cAMP accumulation, adenylate cyclase, or PTH release. 

Figure 7. Dopamine-stimulated adenylate cyclase activity in lysates of bovine parathyroid cells in the absence (Q) or presence (O) of 100 pM. guanosine triphosphate (GTP). (Reproduced with permission from Ref. 11. Copyright 1980, The...

Both calmodulin and a calcium- and calmodulin-dependent phosphodiesterase activity occur in dispersed bovine parathyroid cells (20.). Some of the phenothiazines blocking the dopamine receptor in this tissue inhibit the interaction between calmodulin and the calmodulin-dependent phosphodiesterase. A series of experiments have tested the possibility that calmodulin might play a role in the effects of these agents on the parathyroid gland. The dopamine antagonists examined were weak inhibitors of calmodulin-stimulated phosphodiesterase activity (Table II). 

Effects of dopaminergic antagonists on dopamine-stimulated cAMP accumulation, adenylate cyclase activity, and calmodulin-stimulated phosphodiesterase (PDE) activity in intact bovine parathyroid cells or cellular homogenates. Values for or IC50 are given as uM. NT, not tested. 

All the pharmacological and behavioural effects elicited by dopamine agonists and antagonists in the brain can only be explained if such an interaction occurs at the level of the dopamine receptor (D2 receptor site) the site still remains in search of a function. Bovine parathyroid cells were reported to possess dopamine sites which should be involved in the control of parathormone secretion. However, the very poor pharmacological characterization and the lack of in vivo evidence do not allow to assess the dopaminergic nature of this hormone secretion. Dopamine-sensitive adenylate cyclase is thus not a receptor directly implicated in the dopaminergic neurotransmission it is an enzyme which could have an important role in the control of long term metabolic effects such as the synthesis of neuronal constituents. 

In 1977, Brown et al. (15) reported that dopamine (10-8 M) stimulates by 2-4 fold the secretion of parathormone from dispersed bovine parathyroid cells. 

ADTN and other dopamine agonists mimicked this effect which was antagonized by a- and B-flupenthixol, the a-isomer being 100 times more potent. In a similar way, dopamine caused a rapid 20-30-fold increase in cellular cAMP in dispersed bovine parathyroid cells. The potency of a series of dopaminergic agonists and antagonists on adenylate cyclase activity paralleled the effects of these ligands on CAMP accumulation and parathormone secretion (16). It was concluded that bovine parathyroid cells possess dopamine sites which are involved in the control of parathormone secretion. 

It is incorrect to state that desensitization does not occur in the dopaminergic response of bovine parathyroid cells. We did not, in fact, examine this point directly. Desensitization, however, probably actually does occur at more than one locus in this system. First, the secretory response of the parathyroid cell rapidly becomes refractory to agents such as dopamine and isoproterenol which produce large elevations in cyclic AMP (see ref. 2). Secondly, there is a progressive decrease in cellular cyclic AMP despite the continued presence of dopamine (see Figure 5 in our manuscript) possibly due to desensitization of the receptor-adenylate cyclase compex. 

In Table II of his comments, Dr. Laduron points out differences between bovine and human parathyroid cells. Several of these are in error. Prostaglandins affect both human and bovine parathyroid cells. Phosphodiesterase inhibitors have not been tested directly in human parathyroid cells, although dibutyryl cyclic AMP, which may act in part by inhibiting phosphodiesterase, stimulated PTH release in fragments of human parathyroid glands (6). 

Shoback, D.M., Membreno, L.A., and McGhee, J.G., High calcium and other divalent cations increase inositol trisphosphate in bovine parathyroid cells, Endocrinology 123, 382-389, 1988. 

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