Enhanced Pressor Response to Increased CSF Sodium Concentration and to Central Angiotensin I in Heterozygous α2 Na, K-ATPase Knockout Mice*

Intracerebroventricular (i.c.v.) infusion of NaCl mimics the effects of a high-salt diet in salt-sensitive hypertension, raising the sodium concentration in the cerebrospinal fluid (CSF [Na]) and subsequently increasing the concentration of an endogenous ouabain-like substance (OLS) in the brain. The OLS in turn inhibits the brain Na, K-ATPase, causing an increase in the activity of the brain renin-angiotensin system (RAS) and blood pressure. The Na, K-ATPase α (catalytic) isoform(s) that mediate the pressor response to increased CSF [Na] is (are) unknown, but it is likely that one or more isoforms that bind ouabain with high affinity are involved (e.g., the Na, K-ATPase α2 and/or α3 subunits). We hypothesize that OLS-induced inhibition of the α2 subunit mediates this response. Therefore, a chronic reduction in α2 expression via a heterozygous gene knockout (α2 +/-) should enhance the pressor response to increased CSF [Na]. Intracerebroventricular (i.c.v.) infusion of artificial CSF containing 0.225 M NaCl increased mean arterial pressure (MAP) in both wild-type (+/+) and α2 +/mice, but to a greater extent in α2 +/-. Likewise, the pressor response to i.c.v. ouabain was enhanced in α2 +/mice, demonstrating enhanced sensitivity to brain Na, K-ATPase inhibition per se. The pressor response to i.c.v. angiotensin I but not ANG II was also enhanced in α2 +/vs. +/+ mice, suggesting an enhanced brain RAS activity that may be mediated by increased brain angiotensin converting enzyme (ACE). The latter hypothesis is supported by enhanced ACE ligand binding in the organum vasculosum laminae terminalis (OVLT). These studies demonstrate that chronic downregulation of Na, K-ATPase α2 isoform expression by heterozygous knockout increases the pressor response to increased CSF [Na] and activates the brain RAS. Since these changes mimic those produced by the endogenous brain OLS, the brain α2 isoform may be a target for the brain OLS during increases in CSF [Na], such as in salt-dependent hypertension.