The Role of Plasma Volume, Plasma Renin and the Sympathetic Nervous System in the Posture-Induced Decline in Renal Lithium Clearance in Man

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Abstract
Excretion of Li [CNS depressant] in urine was studied in 2 healthy males while recumbent and while upright, either walking or standing quietly. An oral dose of 24.3 mmol of Li+ was taken as 3 Li2CO3 tablets 13 h before clearance tests. Renal Li clearance decreased and Li fractional reabsorption increased while upright. Standing immersed to the neck in H2O, which prevents the fall in plasma volume upon changing posture from recumbent to upright, prevented the fall in renal Li clearance as well as the rise in Li fractional reabsorption while upright. Oral doses of guanethidine (total dose of 200 mg) or oxprenolol (total dose of 140 mg) taken to prevent high levels of sympathetic nervous system activity and plasma renin, respectively, failed to prevent the fall in renal Li clearance or the rise in Li fractional reabsorption upon changing posture from recumbent to upright. The fall in renal Li clearance and the rise in Li fractional reabsorption upon changing posture from recumbent to upright was related to the fall in plasma volume but not to high levels of sympathetic nervous system activity or plasma renin activity.

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