Parathyroid hormone is not a key hormone in the rapid minute-to-minute regulation of plasma Ca2+ homeostasis in rats.
BACKGROUND
The role of parathyroid hormone (PTH) in the rapid minute-to-minute regulation of plasma Ca2+ (p-Ca2+) was studied in vivo in rats.


METHODS
The rapid calcaemic response to exogenous rat PTH1-34 (16 microg) was examined in normal rats, and the long-term calcaemic response was examined in parathyroidectomized (PTX) rats receiving PTH1-34 for 24 h at 0.2, 0. 4 and 0.8 microg h-1. Acute hypocalcaemia was induced by EGTA for 30 min, and then the rapid recovery of p-Ca2+ was studied for 130 min in normal rats, 24 h after PTX and in PTX rats infused with exogenous rat PTH1-34. The dynamics of the rapid recovery of p-Ca2+ was studied at two additional doses of EGTA.


RESULTS
No rapid calcaemic response was observed in the first 60 min after administration of PTH and no hypocalcaemia was seen for 2 h after acute PTX. This slow effect of PTH suggests that PTH might not be responsible for maintaining the stable p-Ca2+ on a rapid minute-to-minute basis. EGTA induced acute hypocalcaemia in both normal and PTX rats (P < 0.01). In both groups a rapid and similar increase in p-Ca2+ took place 10 min after discontinuing EGTA (P < 0. 05). Within 60 min, p-Ca2+ increased further, independently of the presence of PTH. Infusion of PTH to PTX rats did not affect the rapid recovery of p-Ca2+ (P < 0.05) from EGTA induced hypocalcaemia.


CONCLUSIONS
PTH is not a key hormone in the rapid recovery of p-Ca2+ after induction of hypocalcaemia, but might, however, set the long-term levels of p-Ca2+ maintained by mammalian organisms. The involvement of an as yet unknown factor in the rapid regulation of p-Ca2+ is suggested.
Wang
W
W
Nephrological Department P, Rigshospitalet, University of Copenhagen, Denmark.
Lewin
E
E
Olgaard
K
K
D000818:Animals; D002118:Calcium / Q000097:blood*; D002614:Chelating Agents / Q000494:pharmacology; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000494:pharmacology; D006706:Homeostasis / Q000187:drug effects / Q000502:physiology*; D006996:Hypocalcemia / Q000097:blood* / Q000188:drug therapy*; D008297:Male; D010281:Parathyroid Hormone / Q000097:blood* / Q000494:pharmacology; D016105:Parathyroidectomy; D010446:Peptide Fragments / Q000097:blood* / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors
D002614:Chelating Agents; D010281:Parathyroid Hormone; D010446:Peptide Fragments; D004533:Egtazic Acid; D002118:Calcium