Michal Pravenec

Genetic contamination of Dahl SS/Jr rats. Impact on studies of salt-sensitive hypertension.

The Dahl salt-sensitive rat (SS/Jr) is a widely used animal model of salt-sensitive hypertension. SS/Jr rats are believed to be highly inbred and uniformly sensitive to the hypertensinogenic effects of sodium chloride, but we have recently observed that SS/Jr rats from Harlan Sprague Dawley, Inc, exhibit considerable variability in their blood pressure response to supplemental dietary salt. To test the possibility that commercially available SS/Jr rats are genetically contaminated and therefore no longer fully inbred, we performed molecular genetic studies and blood pressure measurements in several groups of SS/Jr rats purchased from Harlan Sprague Dawley. We found molecular evidence of heterozygosity and/or atypical allelic variants involving loci on at least five different chromosomes. Many of the rats also failed to exhibit a salt-sensitive blood pressure phenotype. We conclude that SS/Jr rats being sold by the only commercial vendor of Dahl rats in the United States are genetically contaminated and resistant to the hypertensinogenic effects of salt. These findings raise serious questions about the interpretation of research conducted with SS/Jr rats obtained from Harlan Sprague Dawley.

Molecular-Based Mechanisms of Mendelian Forms of Salt-Dependent Hypertension Questioning the Prevailing Theory

This critical review directly challenges the prevailing theory that a transient increase in cardiac output caused by genetically mediated increases in activity of the ENaC in the aldosterone sensitive distal nephron, or of the NCC in the distal convoluted tubule, accounts entirely for the hemodynamic initiation of all Mendelian forms of salt-dependent hypertension (Figure 1). The prevailing theory of how genetic mutations enable salt to hemodynamically initiate Mendelian forms of salt-dependent hypertension in humans (Figure 1) depends on the results of salt-loading studies of cardiac output and systemic vascular resistance in nongenetic models of hypertension that lack appropriate normal controls. The theory is inconsistent with the results of studies that include measurements of the initial hemodynamic changes induced by salt loading in normal, salt-resistant controls. The present analysis, which takes into account the results of salt-loading studies that include the requisite normal controls, indicates that mutation-induced increases in the renal tubular activity of ENaC or NCC that lead to transient increases in cardiac output will generally not be sufficient to enable increases in salt intake to initiate the increased BP that characterizes Mendelian forms of salt-dependent hypertension (Table). The present analysis also raises questions about whether mutation-dependent increases in renal tubular activity of ENaC or NCC are even necessary to account for increased risk for salt-dependent hypertension in most patients with such mutations. We propose that for the genetic alterations underlying Mendelian forms of salt-dependent hypertension to enable increases in salt intake to initiate the increased BP, they must often cause vasodysfunction, ie, an inability to normally vasodilate and decrease systemic vascular resistance in response to increases in salt intake within dietary ranges typically observed in most modern societies. A subnormal ability to vasodilate in response to salt loading could be caused by mutation-related disturbances originating in the vasculature itself or in sites outside the vasculature (eg, brain or adrenal glands) that have the capacity to affect vascular function.

Putative candidate genes for blood pressure control in the SHR.BN-RNO8 congenic substrains

Summary The SHR- Lx congenic strain carrying a differential segment of chromosome 8 of BN and PD origin was recently shown to exhibit a significant decrease in blood pressure as compared to the SHR strain. There were two positional candidate genes for blood pressure control mapped to the differential segment: the rat kidney epithelial potassium channel gene ( Kcnj1 ) and brain dopamine receptor 2 gene ( Drd2 ). Bot these genes were separated into SHR.BN-RNO8 congenic substrains. In this communication, we are presenting the assignment of two further putative candidate genes, which might be involved in blood pressure control to the BN/PD differential segment of the SHR- Lx congenic strain. These are: the gene coding for smooth muscle cell specific protein 22 ( Sm22 ) defined by the D8Mcw1 marker and neuronal nicotinic acetylcholine receptor gene cluster, defined by the D8Bord1 marker. Moreover, the glutamate receptor gene Grik4 which also maps to the differential segment of the SHR- Lx should be taken into account. The genetic separation of all these putative candidate genes of blood pressure control is being performed by recombinations and subsequent selection using (SHR×SHR- Lx ) intercross population.