The effects of 1,25-dihydroxyvitamin D-3 deficiency on Ca(2+)-transport and Ca(2+)-uptake into brush-border membrane vesicles from pig small intestine.
BBMV were prepared from duodenal segments of untreated, 1,25-(OH)2D3- or vitamin D-3-treated rachitic piglets and from non-rachitic controls by the Mg2+ precipitation method. The rachitic piglets were offspring from the 'Hannover Pig Strain' which suffer from pseudo vitamin D-deficiency rickets, type I (no renal 1-hydroxylase activity). Initial uptake of Ca2+ (up to 35 s) at low [Ca2+] (between 0.02-0.25 mmol/l) into isolated BBMV consisted of a saturable and non-saturable component. The apparent Vmax of the saturable component was significantly lower in rachitic piglets than in control piglets. In the presence of an inside negative potassium diffusion potential, the difference in uptake extended over at least 15 min. The apparent Km of Ca(2+)-uptake was not influenced by the rachitic condition. Treatment of rachitic piglets with sequential doses of 1,25-(OH)2D3 for three days (1 microgram/day) or with a single dose (2.5 mg) of vitamin D-3 elevated the saturable Ca(2+)-uptake component to values similar to those of control piglets. Addition of 1 mmol/l verapamil to the vesicular suspension inhibited Ca(2+)-uptake in BBMV of control piglets by 40-60% but was without effect on preparations from rachitic piglets. It was concluded from the study that 1,25-(OH)2D3 dependent Ca(2+)-uptake into BBMV constituted a saturable process which can be inhibited by a known Ca(2+)-channel blocking agent. It appears that 1,25-(OH)2D3 increases the number of verapamil sensitive Ca(2+)-transport components in brush-border membranes. The vitamin D-dependent changes in vesicular Ca(2+)-uptake were paralleled by the expression of an active transmural Ca(2+)-transport across the mucosa.
Kaune
R
R
Department of Physiology, School of Veterinary Medicine, Hannover, Germany.
Kassianoff
I
I
Schröder
B
B
Harmeyer
J
J
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002117:Calcitriol / Q000172:deficiency*; D002118:Calcium / Q000378:metabolism*; D004058:Diffusion; D005260:Female; D005947:Glucose / Q000378:metabolism; D007421:Intestine, Small / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D008871:Microvilli / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D013552:Swine; D013556:Swine, Miniature; D014700:Verapamil / Q000494:pharmacology; D014808:Vitamin D Deficiency / Q000378:metabolism*
D014700:Verapamil; D002117:Calcitriol; D005947:Glucose; D011188:Potassium; D002118:Calcium