Physiological Role of the α2-Isoform of the Na, K-ATPase in the Regulation of Cardiovascular Function
Na,K-ATPase is a ubiquitously expressed heteromeric transmembrane protein composed of two essential subunits: α and β. There are four known isoforms of the catalytic α-subunit (α1, α2, α3, and α4), and each isoform displays a unique tissue distribution and expression pattern suggesting a tissue-specific role for the α-subunits. The α2-isoform of the Na,K-ATPase (α2) is the minor isoform of the Na,K-ATPase expressed in the cardiovascular system but is thought play a critical role in the regulation of systolic blood pressure (SBP). However, the organ system/cell type expressing α2 that is required for this regulation has not been fully defined. The present dissertation study uses tissue-specific knockouts of α2 to further define the tissue-specific role of the α2 in the regulation of SBP. To accomplish this, we developed a mouse model utilizing the Cre/LoxP system to generate tissue-specific knockouts of α2 in the vascular smooth muscle and heart using mice expressing Cre recombinase in a smooth muscle and cardiac-specific manner. Using the SM22α Cre we generated a smooth muscle-specific knockout (SMα2-/-) mouse with a 90% reduction in α2-expression in heart and vascular smooth muscle. Interestingly, tail-cuff blood pressure analysis reveals that SBP is unaffected by the smooth muscle-specific knockout of α2. However, the SMα2-/- mice do fail to develop ACTH-induced hypertension, as seen in control mice in response to ACTH treatment (Control=119±7, SMα2-/-=103±2 mmHg). In addition, utlizing the β-MHC Cre we generated a heart-specific knockout (βMHCα2-/-) mouse with a 90% reduction in α2-expression that is specific to the heart. βMHCα2-/- mice exhibit normal basal SBP, and in addition these mice develop ACTH-induced hypertension in response to ACTH treatment similar to control mice (Control=119±3, βMHCα2-/-=117±6 mmHg). Surprisingly, βMHCα2-/- mice display delayed onset of cardiac dysfunction compared to control mice in response to pressure-overload induced by transaortic constriction (TAC), however the βMHCα2-/- mice deteriorated to control levels by nine weeks post-TAC. In summary, this dissertation refines the roles of the α2-isoform of the Na,K-ATPase in the brain, vascular smooth muscle, and heart in the regulation and maintenance of SBP. Though vascular smooth muscle expression of α2 appears to be required for ACTH-induced hypertension, cardiac expression is not. By contrast, cardiac expression of α2 seems to participate in the hypertrophic response to pressure-overload. Interestingly, previous studies indicate that increased salt in cerebrospinal fluid results in an increase in the level of endogenous brain-ouabain, resulting in hypertension that is dependent on the ouabain-binding site of the α2-isoform in the brain. In addition, several reports have shown that an increased level of endogenous brain-ouabain activates the renin-angiotensin system. Taken together these results suggest that expression of α2 in the brain is required for the basal regulation of SBP. Furthermore, these data support the hypothesis that the tissue localization of the α2-isoform determines its unique roles in the regulation and maintenance of SBP.
Biochemistry, Microbiology, Genetics, Adrenocorticotropic hormone, Endogenous cardiac glycosides, Hypertension, Ouabain, Systolic Blood Pressure, Transaortic constriction
Networked Digital Library of Theses and Dissertations