Hypoxia and Cell Signalling: Cardiovascular Remodelling, Glucose Homeostasis and Role of Calcium Channel Blocker (Cilnidipine)

Abstract

Objective: The present study was undertaken to evaluate the cardiovascular autonomic functions and hypoxia signalling molecules after chronic hypoxia exposure and their impact on cardiovascular remodelling and glucose homeostasis and the role of cilnidipine, a dual L/N type calcium channel blocker. Further, the impact of unilateral common carotid artery occlusion on brain histopathology in experimental animals preconditioned to chronic hypoxia was assessed. Methods: Twenty-four adult male Wistar strain albino rats (Rattus Norvegicus) were randomly allocated into four groups. Group 1: control; group 2: chronic hypoxia (10% O2 and 90% N2) exposed for 21 days; group 3: cilnidipine treated (2.0mg/kg/day); group 4: chronic hypoxia exposed (10% O2 and 90% N2) and cilnidipine treated (2.0mg/kg/day) for 21 days. All the experimental animals were subjected to gravimetry and % body weight gain was calculated after 21 days. Haematological parameters like RBC count (million cells/mm3), Hb (g/dl) and Hct (%) were estimated. HRV analysis was done to assess cardiovascular autonomic balance. Hemodynamic parameters like heart rate and blood pressure were recorded. Oxidative stress and antioxidant defence were assessed by estimating MDA, vitamin C, vitamin E in the serum and MDA in heart, lung and liver tissue homogenate. Hypoxia signalling molecules like VEGF, NOS3 and NO were estimated in the serum. Glucose homeostasis was evaluated by estimation of fasting plasma glucose, oral glucose tolerance test (OGTT), fasting serum insulin. HOMA-IR was calculated as an index of insulin resistance. Cardiovascular remodelling was studied by calculation of cardiosomatic index, and histopathological examination of H&E stained sections of the ventricles, intramyocardial coronary artery, elastic artery and muscular artery. Further, the normalised wall index (NWI), was calculated for the coronary artery. Lipid profile was assessed. Histopathological examination of the lung and liver were also done. To study the impact of unilateral common carotid artery (CCA) occlusion on brain pathophysiology in chronic hypoxia pre-exposed rats, the experimental animals were randomly assigned to one of the three groups: group 1: sham-operated; group 2: normoxia (21% oxygen), left CCA occlusion for 75 minutes and subsequent reperfusion for 12 hours; group 3: Hypoxia (10% O2) pre-exposed for 21 days prior to left CCA occlusion for 75 minutes and reperfusion for 12 hours. After reperfusion for 12 hours, the experimental animals were assessed for neurologic deficits and then were sacrificed. The brain was dissected and subjected to histopathological examination. Results: Chronic hypoxia resulted in lower % body weight gain, elevated hematocrit. HRV analysis revealed sympathetic overactivity and shift in the sympathovagal balance. Cardiovascular hemodynamics revealed decreased heart rate, increased mean arterial pressure (MAP). There were disturbances in oxidant-antioxidant balance indicating oxidative stress. VEGF, NOS3 were markedly elevated and NO significantly reduced. Glucose homeostasis was disturbed with increased fasting plasma glucose and HOMA-IR. HOMA-IR and fasting plasma glucose were positively correlated with the LF/HF ratio. There were features suggestive of cardiovascular remodelling. Further, NWI of the coronary artery was positively correlated with LF/HF ratio, heart tissue MDA, serum MDA and VEGF. Cilnidipine treatment was able to 1) control sympathetic overactivity 2) reduce MAP 3) decrease oxidative stress 4) increase the bioavailability of NO 5) improve glucose homeostasis and 6) ameliorate cardiovascular remodelling resulting from chronic hypoxia exposure. Brain histopathology in rats pre-exposed to chronic hypoxia and subjected to unilateral left common carotid artery occlusion demonstrated a reduction in brain oedema, a smaller infarct volume and lesser neurological deficits. This study demonstrates that rats pre-exposed to chronic hypoxia could have reduced brain injury after focal ischemia as compared to normoxic (hypoxia unexposed) experimental animals. Conclusion: The present study demonstrates that chronic hypoxia exposure is accompanied by altered hypoxia signalling mechanism, impaired oxidant/antioxidant balance and shift in the sympathovagal balance towards the increased sympathetic activity. These alterations further proceed to impact glucose homeostasis and induce cardiovascular remodelling. Cilnidipine, owing to its dual L/N type calcium channel blocking properties with additional antioxidant potential probably has a beneficial role in ameliorating chronic hypoxia-induced cardiovascular remodelling, disturbances in glucose homeostasis suggesting a possible therapeutic use of cilnidipine against hypoxia-related pathophysiology.