Vasopressin-independent alterations in renal water excretion in quadriplegia.

Postural effects on water excretion are known to be increased in patients with cervical spinal cord injury and may result in marked impairment of the ability to excrete a water load, especially in erect posture. Both vasopressin-dependent and vasopressin-independent mechanisms have been implicated. To assess the roles of these mechanisms and further identify the factors involved in the renal response to erect posture, sustained water loading studies were performed on 11 quadriplegic subjects and 9 healthy control subjects, supine and erect (sitting). Renal blood flow was assessed by p-aminohippurate clearance (CPAH) measurements in 7 quadriplegic and 5 control subjects. During maximal water diuresis, plasma vasopressin concentrations were reduced to unquantifiable levels in all subjects. Osmolar clearance, free water clearance (CH2O), and distal delivery of filtrate (DDF) were all lower in quadriplegic than in control subjects, supine and erect. The relationship between CH2O and DDF was the same in quadriplegic as in control subjects and was not altered by change in posture in either group. Creatinine clearance and CPAH were lower in erect than in supine posture in quadriplegic subjects but not in control subjects. We conclude that impairment of water excretion in stable normonatremic quadriplegic subjects can be attributed primarily to vasopressin-independent mechanisms involving reduced filtrate delivery to diluting segments of the renal tubules rather than to resistance to normal suppression of vasopressin release.