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Six young (3 yr old) and six old (8 to 11 yr old) cows were used to examine renal function, KCl and sodium citrate effects on Mg clearance rate and the relation of parathyroid hormone (PTH) to Mg reabsorption. The treatments were 1.5 g KCl/kg body weight (BW) and 1.5 g Na citrate/kg BW. Inulin and p-aminohippuric acid (PAH) clearance rates were used as measures of glomerular filtration rate (GFR) and effective renal plasma flow (ERPF), respectively, in control cows on each treatment day. Creatinine clearance rate was also used as a measure of GFR in all cows. Magnesium loading (2.4 mg/kg BW) through continuous intrajugular infusion of MgCl2 facilitated the measurement of changes in Mg clearance rate and reabsorption due to treatment. Inulin and creatinine clearance rates were not different as measures of GFR, and there were no differences in ERPF between young and old cows. Old cows had lower (P less than .05) GFR, creatinine to PAH clearance ratios and tubular reabsorption of Mg than young cows. Intraruminal infusion of KCl and Na citrate increased (P less than .05) the GFR. Potassium chloride increased plasma K concentration (P less than .001), plasma K clearance rate (P less than .002), plasma Na clearance rate (P less than .03), plasma Mg concentration (P less than .05), net tubular reabsorption of Mg (P less than .02), maximum tubular reabsorption of Mg (P less than .002) and plasma PTH concentration (P less than .03). Plasma Mg clearance rate was increased only by Na citrate (P less than .05). These results suggest a possible relationship between age, renal function, Mg antagonists and PTH in Mg homeostasis and in the etiology of hypomagnesemic tetany.

Eighteen preruminating calves obtained at approximately 38 d of age were used to determine whether the ruminant insulin response to propionate is an inherent response or one that is acquired because of ruminal propionate production. A liquid milk replacer diet was fed alone or with isocaloric additions of propionate [50 mmol . (kg BW . 75-1 . d-1] or glucose. Both additions caused a strong insulin response, but the response by propionate-fed calves appeared to be independent of plasma glucose concentrations during the early post-feeding hours. After receiving the experimental diets for 18 d, all calves were given an iv infusion or propionate [.035 mmol propionate . (kg BW . 75-1 . min-1 for 20 min]. All calves demonstrated a plasma insulin rise and glucose decline due to the infusions, but glucose-fed and propionate-fed calves appeared less sensitive to the iv propionate than the calves fed only milk replacer.

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