Loss of Hepatic Vitamin A Reserve Lowers Cardiac Function and Prevents Cardiac Calcium Overload Under Stress Conditions

Abstract

Vitamin A is an essential nutrient for both cardiac development and repair following injury. Vitamin A is primarily stored in the liver by the lecithin retinol acyltransferase (LRAT) enzyme and the liver secretes 30-40% of its vitamin A stores following myocardial injury. Previously, we found that mice lacking LRAT (LRAT KO), and thus lacking hepatic Vitamin A stores, preserve cardiac function following myocardial infarction compared to wild type (WT) control mice. Our finding contradicts other reports that suggest vitamin A is needed for cardiac repair. A gene microarray of left and right ventricular tissue identified changes in multiple calcium and ion channels in LRAT KO mice. All of our findings led us to hypothesize that LRAT KO mice have impaired cardiac calcium handling and that therefore the heart may be protected from calcium overload during an ischemic event. Hearts from LRAT KO and WT mice were perfused in the Langendorff mode to measure systolic and diastolic parameters. Heart function was determined at baseline, at high calcium levels (4 mM), and during adrenergic stimulation with dobutamine (300 nM). At baseline, LRAT KO and WT mice showed no significant differences in any of the cardiac function parameters. However, when stressed with dobutamine LRAT KO hearts had a 34% decrease in diastolic pressure (KO 4329±591 mmHg vs. WT 6103±508 mmHg, n=6-7, p<0.01) and a 25% reduction in systolic pressure (KO 8674±1365 mmHg vs. WT 11103±872 mmHg, n=6-7, p<0.05). Coronary flow, heart rate, rate pressure product and developed pressure where not significantly altered. When stressed with 4 mM extracellular calcium, we found a strong impairment in all contractile function parameters in LRAT KO compared to WT hearts. Most affected were diastolic relaxation (ndP/dt) with a decrease of 43% (KO 2819±281 mmHg/svs. WT 4347±383 mmHg/s, n=6-7,p<0.01) and contractility (pdP/dt) with a 40% reduction (KO 5013±444 mmHg/s vs. WT 7507±790 mmHg/s, n=6-7, p<0.05). Western blot analysis of cardiac tissue identified a reduction in calmodulin (p<0.05) and calsequestrin 2 in the LRAT null hearts supporting that blunted calcium handling is occurring in these hearts. In summary, hearts from mice lacking vitamin A stores are unable to increase cardiac function under stress conditions. This finding is more pronounced with high calcium stress than with adrenergic stimulation. These results support our hypothesis that LRAT KO mice are potentially protected from calcium overload during ischemic injury.