Investigation of different blood pressure regulating systems in kininogen-deficient BNK rats
The kininogen-deficient BNK rats were shown to progressively develop salt-sensitive hypertension by accumulation of Na+. It was believed that the kininogen-deficiency is responsible for the defective NaCl excretion leading to a cardiovascular damage. In addition to the kallikrein-kinin system (KKS), endothelin (ET) and aldosterone have important implications in the regulation of salt homeostasis. To precisely characterize the relations between the KKS, ET and aldosterone male BN and BNK rats were given standard or high salt diet in presence or absence of spironolactone for 10 days. Besides for experiments with metabolic cages the rats were characterized by tail-cuff measurements of blood pressure. Kininogens, bradykinin (BK), kallidin-like peptide (KLP), ET-1, deoxycorticosterone (DOC), corticosterone and aldosterone were measured by specific RIAs in plasma and 24h urine. Corticoids were additionally determined in brain tissue. Activity of plasma and urinary kallikrein was measured with chromogenic substrates. Expression analysis of adrenal steroidogenic enzymes 11ß-hydroxylase and aldosterone synthase was performed by real-time RT-PCR in LightCycler. Functional cardiac parameters were assessed in isolated hearts perfused according to Langendorff technique. Acute effects of 30min global ischaemia with or without previous ischaemic preconditioning (IPC) were investigated. Myocardial damage was judged by the creatine kinase activity in the coronary effluent measured with a specific kit. BNK rats are characterized by plasma kininogen deficiency that is reflected in lower levels of plasma kallikrein activity and consequently in lower plasma BK and KLP levels. The renal KKS of BNK rats was almost identical to that of wild type BN rats. Plasma and urinary KLP was found to be the major kinin responsible for most physiological effects mediated by the B2 receptor in both rat strains. We found that the deleterious effects of salt diet in BNK rats are in fact attributable to enhanced mineralocorticoid action. The renal mineralocorticoid receptor (MR) displayed an overexpression and several mutations that might be responsible for altered affinity and responses toward aldosterone and other ligands, e.g. DOC and corticosterone. Moreover, the BNK rats have increased levels of plasma and urinary ET-1, which may contribute to the deleterious effects of Na+ during high salt diet. In vitro investigation of ischaemic preconditioning in isolated hearts revealed no significant findings. Missing plasma components are necessary for the overall protective effects of IPC. Nevertheless, long-lasting effects like attenuation of plasma kinins or antagonism of the MR seemed to be of importance for the regulation of cardiac function. The absence of plasma KLP during high salt diet was found to be responsible for the enhanced deleterious effects of Na+ leading to cardiac hypertrophy. In BNK rat hearts aldosterone was found to be responsible for the higher heart rate and increased contractility besides for the impairment of coronary flow. Aldosterone is also involved in the acute response to ischaemia and consequently in response to IPC. In case of an enhanced mineralocorticoid action like in BNK rats aldosterone may account for the increased sensitivity to ischaemia whereby attenuating the protective effect of IPC. We conclude that in BNK rats the higher blood pressure, increased cardiac sensitivity to ischaemia and diminished effects of IPC are not attributable to the kininogen deficiency but rather caused by the enhanced mineralocorticoid action.
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