Cerebrospinal fluid pressure of conscious rats after venous constriction at the right atrium

Thoracic shifts of blood stimulate diuresis and natriuresis during spaceflight. The available literature is not conclusive as to whether thoracic afferent neurons are essential for this response. Possibly, an acute elevation in cerebrospinal fluid pressure (CSF-p) activates central compensatory mechanisms. This is because central venous pressure is elevated by thoracic blood shifts and may reduce the pressure gradient for drainage of CSF into the venous sinuses. We tested whether rats with constriction of the venous return at the level of the heart (0.4 mm maximum diameter) had CSF-p different from sham-operated controls. CSF-p in the immediate postoperative period, as well as 1 and 10 days after surgery, were within normal limits and did not differ (p greater than 0.05). Blood collected at the end of the experiment showed no group differences (p greater than 0.05) in the hematocrit, or concentrations of sodium, potassium or vasopressin. Thus, changes in CSF-p, per se, appear to be insufficient to explain the cardiovascular or salt/water balance readjustments observed in spaceflight. It is likely that compensatory systems are highly redundant.