Renal tubular sodium handling and plasma atrial natriuretic peptide, renin activity and aldosterone in untreated men under normal living conditions.

The associations between renal tubular sodium handling and plasma levels of atrial natriuretic peptide, renin activity and aldosterone were studied in 295 untreated men under normal living conditions. The renal clearance of ingested lithium was used as a marker of proximal tubular sodium handling. Plasma atrial natriuretic peptide was inversely related to creatinine clearance (r = -0.148, P less than 0.01) and directly and significantly related to the overall fractional excretion of sodium (r = 0.213, P less than 0.001) and to distal (r = 0.151, P less than 0.01) fractional sodium excretion. Plasma renin activity was inversely related to sodium excretion at both proximal (r = -0.145, P less than 0.05) and distal (r = -0.236, P less than 0.001) tubular site, whereas plasma aldosterone was significantly and inversely related to distal sodium excretion only (r = -0.305, P less than 0.001). The association between plasma atrial natriuretic peptide and distal sodium excretion in a large sample of men under normal living conditions supports the view of a possible tubular effect of the hormone of the overall control of sodium excretion in man.

Erythrocyte volume and blood pressure in a cross-sectional population-based study.

A reduction in mean erythrocyte volume has been reported in some strains of genetically hypertensive rat, and more recently it has been suggested that a similar alteration might be found in human essential hypertension. The relationship between erythrocyte volume and blood pressure was therefore studied in a random sample of an untreated male working population (n = 317; age 45.1 +/- 6.4 years, mean +/- s.d.). Neither systolic nor diastolic blood pressures were found to be related to erythrocyte volume (r = 0.022 and r = -0.014, respectively); in fact, erythrocyte volume was not different across quintiles of blood pressure. Smokers (n = 171) had lower blood pressure and a greater erythrocyte volume than non-smokers or ex-smokers (n = 144; 91.6 +/- 4.7 versus 88.2 +/- 5.5 fl; P less than 0.001), and heavy drinkers (greater than 110 g ethanol/day) had higher blood pressure and a greater erythrocyte volume compared with the rest of the study population (P less than 0.01). However, after adjustment of erythrocyte volume for these two potentially confounding factors, again no statistical association was found with blood pressure. The present study, therefore, does not support the hypothesis of a negative association between erythrocyte volume and blood pressure, whereas it confirms that the smoking habit and habitual alcohol intake are strong determinants of erythrocyte volume.

Insulin-induced increase in heart rate and its prevention by propranolol.

Acute hyperinsulinaemia in the absence of changes in blood glucose increases heart rate in man. Animal studies have suggested that beta-adrenergic blockade does not prevent the insulin-induced increase in heart rate. The aim of the present study was to investigate the acute effect of insulin on heart rate and blood pressure in non diabetic subjects and, in particular, to determine whether beta-adrenergic receptor blockade would significantly influence the effect. On separate days 9 healthy young volunteers were pretreated with either 80 mg propranolol or placebo p.o. After a 60-90 min period of heart rate and blood pressure stabilization, a placebo injection was given intravenously and heart rate and blood pressure were then monitored every 5 min. After 30 min insulin Actrapid MC 0.2 IU/kg body weight was given i.v. A 20% glucose infusion was given to maintain blood glucose at its fasting level. After insulin administration, a rapid and statistically significant increase in heart rate was observed when the patients were pretreated with placebo; pretreatment with propranolol completely prevented this effect. Serum insulin levels were significantly higher than baseline at all times and there was no significant change in blood glucose. The results are consistent with the hypothesis that the insulin-induced increase in heart rate in man may result from stimulation of cardiac sympathetic activity.

Erythrocyte sodium/lithium countertransport and renal lithium clearance in a random sample of untreated middle-aged men.

1. It has been proposed that the enhanced erythrocyte Na+/Li+ countertransport observed in many patients with essential hypertension could be a marker of abnormal renal proximal tubular function. We thus investigated the relationship of blood pressure and Na+/Li+ countertransport to an index of proximal tubular function such as renal Li+ clearance. 2. The study was carried out in a sample of 299 untreated male subjects (aged 21-59 years) randomly selected from a population at work. Na+/Li+ countertransport was measured in a representative sub-group of 176 men. 3. We did not detect statistically significant correlation of either blood pressure or Na+/Li+ countertransport (Vmax) with fractional excretion of Li+, while confirming the existence of a significant continuous association of blood pressure and body mass index with Na+/Li+ countertransport (P less than 0.01). 4. A sub-sample of 57 participants belonging to the lowest or the highest quintiles of Na+/Li+ countertransport distribution repeated the Li+ clearance study after moderate Na+ restriction. 5. Although fractional excretions of Na+ and Li+ were reduced on the low Na+ diet (both P less than 0.001), they did not differ significantly between groups. 6. Our results are at variance with the findings of a recent case-control study in a young age group and suggest that further studies are necessary before a conclusion can be drawn as to the suitability of Na+/Li+ countertransport as a marker of Na+ reabsorption in the renal proximal tubule.

Calcium-channel blockers and sodium intake: a controlled study in patients with essential hypertension.

Thirteen patients with mild to moderate essential hypertension and whose average supine blood pressure with no treatment was 165/104 mmHg were studied as inpatients for 3 consecutive one-week periods on different sodium intakes. On the last day of each dietary period, they received a single, 20-mg nitrendipine tablet and blood pressure was monitored every 10 minutes for 2 hours after drug administration. Nitrendipine significantly lowered blood pressure independently of the level of sodium intake, and the maximum blood-pressure lowering effect was achieved approximately 1 hour after the dose. The blood-pressure lowering effect of nitrendipine was greater on high sodium intake as compared to low sodium intake (p less than 0.02), and it was also greater with higher initial blood pressures. However, the sodium-related effect on blood pressure was, at least in part, independent of the pretreatment blood pressure. These results suggest that calcium antagonists, such as nitrendipine, are effective in reducing blood pressure in patients with essential hypertension and could be drugs of choice in those who are unable to restrict their salt intake.

Comparison of variability of urinary sodium, potassium, and calcium in free-living men.

The variability of 24-hour urinary sodium, potassium, and calcium excretion was studied in a sample of 22 Neapolitan men with mild blood pressure elevation. On 5 days within a 1-month period, 24-hour urine specimens were collected by each subject. The estimated ratio of intraindividual-to-interindividual variance was 1.12 for urinary sodium, 2.46 for urinary potassium, and 0.52 for urinary calcium. Based on these values, five 24-hour urine collections are necessary to reduce to less than 10% the diminution of the correlation coefficient between urinary sodium and another related variable; this number is substantially lower than that found in previous studies in a North American population sample, but similar to the one reported for Chinese population samples. Ten urinary collections are needed for potassium excretion. A different and more favorable situation is observed for 24-hour calcium excretion, as only two collections are necessary for the adequate characterization of individuals in a population.

Use of fractional lithium clearance in clinical and epidemiological investigation: a methodological assessment.

1. The fractional clearance of lithium (FCLi) has been validated in the rat under controlled experimental conditions as a reliable indicator of sodium and water handling in the proximal tubule. The purpose of the present study was to evaluate some key methodological aspects related to the use of the FCLi in clinical and epidemiological investigation. 2. FCLi was determined in healthy normotensive, or in some cases, in borderline/mild essential hypertensive subjects, by a morning urine collection obtained between 09.00 and 13.00 hours after a 300 mg oral lithium carbonate load (= 8.1 mmol of elemental lithium). 3. The ratio of intra-individual to inter-individual variance of FCLi, measured in free-living subjects on unrestricted diet, was shown to be low enough (0.33) to allow adequate characterization of individuals in a population with a single measurement, or at most with two (compared with at least four measurements needed to characterize the fractional excretion of sodium). 4. The remarkable influence of dietary sodium intake on FCLi, demonstrated under metabolic ward conditions, might explain a major portion of the observed intra-individual variability. 5. At the dosage employed in the present study, oral lithium administration did not affect the renal handling of sodium, potassium or calcium. Likewise, it did not induce any change in a series of 17 metabolic parameters and indicators of renal and liver function. 6. It is concluded that the FCLi may be a safe and useful tool for the clinical and epidemiological investigation of renal sodium and water handling. The possibility of a confounding effect of dietary sodium intake, however, should be kept in mind.

Leisure time physical activity and blood pressure in schoolchildren.

The relation between blood pressure and habitual physical activity during leisure time was investigated in a random sample of 272 sixth grade schoolchildren (mean age, 11.3 years) who entered secondary school in 1983 in Marano di Napoli, a suburb of Naples, Italy. Blood pressure, pulse rate, and anthropometry were measured with standardized techniques. Physical activity was evaluated by a questionnaire outlining four levels of physical activity during leisure time. The study provided evidence that a low level of physical activity during leisure time in 11-year-old children is associated with higher systolic blood pressure independent of sex, age, and adiposity. These findings may be relevant to programs of primary prevention of arterial hypertension in early life.