Plasma vasopressin and aldosterone responses to oral and intravenous saline rehydration

This investigation examined plasma arginine vasopressin (AVP) and aldosterone (Ald) responses to 1) oral and intravenous (IV) methods of rehydration (Rh) and 2) different IV Rh osmotic loads. We hypothesized that AVP and Ald responses would be similar between IV and oral Rh and that the greater osmolality and sodium concentration of a 0.9% IV saline treatment would stimulate a greater AVP response compared with a 0.45% IV saline treatment. On four occasions, eight men (age: 22.1 ± 0.8 yr; height: 179.6 ± 1.5 cm; weight: 73.6 ± 2.5 kg; maximum O2consumption: 57.9 ± 1.6 ml · kg−1 · min−1, body fat: 7.7 ± 0.9%) performed a dehydration (Dh) protocol (33°C) to establish a 4–5% reduction in body weight. After Dh, subjects underwent each of three randomly assigned Rh (back to −2% body wt) treatments (0.9 and 0.45% IV saline, 0.45% oral saline) and a no Rh treatment during the first 45 min of a 100-min rest period. Blood samples were obtained pre-Dh, immediately post-Dh, and at 15, 35, and 55 min post-Rh. Before Dh, plasma AVP and Ald were not different among treatments but were significantly elevated post-Dh. In general, at 15, 35, and 55 min post-Rh, AVP, Ald, osmolality, and plasma volume shifts did not differ between IV and oral fluid replacement. These results demonstrated that the manner in which plasma AVP and Ald responded to oral and IV Rh or to different sodium concentrations (0.9 vs. 0.45%) was not different given the degree of Dh (−4.5% body wt) and Rh and amount of time after Rh (55 min).

Combined effects of age and exercise on thromboxane B2 and platelet activation

Nine healthy young (27.8 +/- 0.8 yr old, YM) and nine healthy older men (55.4 +/- 1.3 yr old, OM) ran on a treadmill at 70–75% of maximal O2 consumption for 30 min to determine the combined effects of age and acute exercise on plasma thromboxane B2 (TxB2) and beta-thromboglobulin (beta-TG). Blood samples (10 ml) were collected in tubes containing 0.5 ml of 4.5 mM EDTA, 30 mM acetylsalicylic acid, and 1 microM prostaglandin E1 after 15 min of rest, immediately after exercise, and at 30 min of recovery. Concentrations of TxB2 and beta-TG were determined by radioimmunoassay. Samples were adjusted for hemoconcentration and changes in platelet count not accounted for by plasma volume shifts. Data were analyzed with repeated-measures analysis of variance and Tukey's post hoc technique. Resting TxB2 was 53.3 +/- 9.6 and 79.0 +/- 18.2 pg/ml for the YM and OM, respectively. beta-TG at rest was 152.5 +/- 13.9 and 114.0 +/- 10.9 ng/ml for the YM and OM, respectively. No significant age group or exercise-induced differences for TxB2 or beta-TG were found immediately after exercise. TxB2 in the OM (101.7 +/- 16.4 pg/ml) 30 min after exercise was significantly (P = 0.05) higher than that in the YM (54.4 +/- 6.2 pg/ml). beta-TG values 30 min after exercise were not significantly different: 183.3 +/- 26.9 and 169.9 +/- 17.0 ng/ml in the OM and YM, respectively. These data suggest that OM may experience greater increases in TxB2 than YM 30 min after exercise and may be more predisposed to platelet activation.