title: cAMP-stimulated rise of [Ca2+]i in rabbit connecting tubules: role of peritubular Ca.
abstract: Parathyroid hormone (PTH) increases cytosolic free Ca concentration ([ Ca2+]i) by mechanisms that depend on extracellular Ca in both cultured renal proximal tubules and isolated rabbit connecting tubules (CNTs). In CNTs 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) mimics this action, implicating cAMP as a second messenger, and part of the rise, due to increased luminal membrane Ca entry, is likely related to Ca absorption. In cultured proximal tubules the rise in [Ca2+]i, presumably mediated by increased Ca entry across the basolateral plasmalemma, activates gluconeogenesis and shortens microvilli. In the present study we examined cAMP-mediated Ca entry across the basolateral membranes of CNT cells, an effect potentially related to cell activation. Single CNTs were dissected from rabbit kidneys and loaded with fura-2. [Ca2+]i was measured by dual-wavelength excitation during perfusion of isolated segments in vitro. With 1.8 or 2.0 mM Ca in the lumen and the bath, suffusate 8-BrcAMP increased [Ca2+]i within minutes in a dose-dependent fashion. The increase persisted as long as 8-BrcAMP was present and reversed on its withdrawal. With 0.1 microM Ca in the lumen and the bath, 8-BrcAMP, but not ionomycin, failed to increase [Ca2+]i, implying that extracellular Ca is the major source. In tubules perfused with 2 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid to eliminate luminal Ca, but suffused with 1.8 or 2.0 mM Ca, 8-BrcAMP increased [Ca2+]i (though less so than with Ca in the lumen), implying Ca entry across basolateral cell membranes. This rise in [Ca2+]i was attenuated markedly by the presence of 50 microM LaCl3 in the bath.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Medicine, Michael Reese Hospital and Medical Center, Chicago, Illinois.
authors.forename: J E
authors.initials: JE
authors.lastname: Bourdeau
authors.forename: B K
authors.initials: BK
authors.lastname: Eby
mesh_terms: D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology / Q000502:physiology; D000242:Cyclic AMP / Q000494:pharmacology*; D005110:Extracellular Space / Q000378:metabolism; D007684:Kidney Tubules / Q000378:metabolism*; D007685:Kidney Tubules, Collecting / Q000378:metabolism*; D009994:Osmolar Concentration; D011817:Rabbits
chemical_list: D015124:8-Bromo Cyclic Adenosine Monophosphate; D000242:Cyclic AMP; D002118:Calcium