Hyponatremia: answered and unanswered questions

Posture-related tachycardia in older patients with hyponatremia

Hyponatremia (HN) is the commonest electrolyte abnormality in elderly patients. Its etiology in this setting is poorly understood. In this study, the authors aim to compare the hemodynamic and hormonal responses of a group of older patients with a predisposition to HN with a group of age-matched controls. We assessed hemodynamic and hormonal responses to postural challenge in 15 patients over age 65 with serum sodium concentrations of less than 130 mM (mean 128.7 mM) and 15 age-matched controls with normal sodium concentrations. Patients remained recumbent for 1 h and stood for the second. Blood was drawn at baseline and at 15 min intervals. Blood pressure (BP) and pulse rates (PR) were monitored electronically. Plasma arginine vasopressin (AVP), atrial natriuretic peptide (ANP), renin and aldosterone were determined periodically during the study period. No difference in BP between groups was noted. PR increased significantly in the HN group only within 3 min of standing (from 71 +/- 4 to 86 +/- 5, P<0.01) and remained significantly higher than controls until 90 min (87 +/- 5 vs. 69 +/- 4, P<0.01). While plasma AVP levels increased significantly following 30 min standing and remained elevated for both HN and control groups, it did not differ significantly between the two. Baseline plasma ANP levels were significantly higher in HN patients compared with controls and remained significantly higher (P<0.05) throughout the study. There was no significant difference in plasma renin or aldosterone levels between groups during the study period. We have demonstrated differing autonomic and hormonal responses to orthostatic challenge between HN patients and age-matched controls. Water retention due to the syndrome of inappropriate anti-diuretic hormone secretion (with reset osmostat) may lead to raised ANP levels in this older cohort of patients. Further physiological studies are required to clarify the precise mechanism of these varying responses.

Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. I. Characteristics of transport mechanisms

The characteristics of amino acid efflux from pre-loaded cells in incubated slices of rat cerebral cortex have been investigated under basal conditions (isosmotic media, 315 mosmol/kg) and following mild hyposmotic shock (265 mOsmol/kg). Rates of efflux have been correlated with the extent of cell swelling in hyposmotic media. Hyposmolality accelerated the slow phase of cellular efflux of L-aspartate (+ 29%), gamma-aminoisobutyric acid (GABA) (+ 38%), L-glutamate (+ 28%) and glycine (+ 26%). The anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-sulfonic acid (DIDS, 25 or 100 microM) as well as trifluoperazine (TFP, 25 microM), an inhibitor of calmodulin activation, both retarded efflux in hyposmotic media, with associated cell swelling (increase in slice non-inulin space). The effects of DIDS and TFP were not additive. N-Ethylmaleimide (NEM, 100 microM) significantly retarded the efflux of neutral amino acids, with cell swelling: these effects were less pronounced in cells loaded with acidic amino acids. It is concluded that the hyposmotically-activated efflux of carboxylic amino acids, and associated cell swelling limitation, requires calmodulin activation and the presence of free sulfydryl groups.