Objectives of this experiment were to determine the length of exposureto an acidogenic diet (ACD) to elicit an increased response to parathyroid(PTH)-induced changes in blood Ca in prepartum cows. The hypothesiswas that cows have increased PTH responsiveness within 3 d offeeding an ACD. Ten parous Holstein cows at 242 ± 7 d of gestationwere blocked by lactation (1 or > 1) and pretreatment DMI and, withinblock, they were assigned randomly to an alkalogenic (ALKD; DCAD= +209 mEq/kg DM; n = 5) or an ACD (DCAD = −168 mEq/kg DM;n = 5) on experiment d 0. Water and DMI were measured and bloodsampled daily. Urine was sampled every 3 h for 36 h and then daily.The PTH challenges were performed on d 3, 8, and 13. Cows received0.05 mg PTH/kg BW i.v. every 20 min for 9 h to mimic the pulsatilerelease of endogenous PTH. Jugular blood was sampled at 0 h, andhourly thereafter until 10 h, and at 12, 18, 24, 36, and 48 h relative tothe challenge. Blood acid-base measures and concentrations of ionizedCa (iCa) were evaluated. Results were available for the first challengeon d 3 and data were analyzed by ANOVA with mixed models withSAS. Cows fed ACD had lower (P < 0.01) blood pH (7.382 vs. 7.429 ±0.005), base excess (−2.4 vs. 4.3 ± 0.5 mM), and bicarbonate (22.8 vs.28.5 ± 0.4 mM) within 24 h of the experiment compared with cows fedALKD. Urine pH decreased (P < 0.01) by 15 h of feeding ACD (7.32 vs.8.18 ± 0.17), and differences increased by 24 h (6.46 vs. 8.10 ± 0.17).Blood iCa increased (P < 0.01) in ACD compared with ALKD by d3(1.28 vs. 1.22 ± 0.01 mM). During the PTH challenge on d3, cows fedACD had a higher (P < 0.01) concentration of blood iCa than cows fedALKD (1.42 vs. 1.33 ± 0.01 mM). Nevertheless, the increment in iCain the first 36 h after the challenge, relative to baseline at 0 h, did notdiffer between treatments (ACD = 0.16 vs. ALKD = 0.15 ± 0.01 mM).Diet-induced metabolic acidosis occurred within 24 h of treatment;however, an increase in blood iCa concentration was observed after 3d of metabolic acidosis. Blood iCa response to a PTH challenge did notdiffer between treatments on experiment d 3.
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