Thyroid Hormone Deficiency Adversely Affects Cardiomyocyte T‐tubule Structure and Function

Abstract

Thyroid hormone deficiency causes adverse changes in cardiomyocyte shape leading to increased wall stress, chamber dilatation and in the long‐term can result in heart failure. We hypothesized that thyroid hormones regulate cardiac contractile function in part by maintaining T‐tubule (TT) structures and efficient excitation‐contraction (EC)‐coupling.MethodsFemale Sprague‐Dawley rats (aged 12 wks) were surgically thyroidectomized (Tx) or were maintained as euthyroid (Eu) controls (6 rats/group). The study was terminated after 12 mo. at which time echocardiographic and hemodynamic measurements were recorded. Ventricular myocytes were isolated by collagenase digestion and plated onto laminin‐coated glass coverslips for TT analysis or were analyzed immediately for contractile and calcium dynamic using videobased edge detection of sarcomere length changes coupled to a fluorescence imaging system (IonOptix). Live cells were labeled with wheat germ agglutinin‐coupled to Alexa‐Fluor (WGA‐488) to image the sarcolemma and TT by confocal microscopy. Multiple z‐stack images were captured per cell and TT periodicity was analyzed using an algorithm to calculate periodicity based on the peak amplitude in the Fourier spectrum of the image at the TT frequency, or TT power. This software program is available as TTorg plugin on ImageJ.ResultsEchocardiography and in vivo hemodynamic measurements showed significantly reduced systolic and diastolic function in Tx vs euthyroid rats. In response to 1 Hz field stimulation at 4V, isolated cardiomyocyte contractile cycles showed significant dysfunction in Tx vs Eu cardiomyocytes: slower max contraction velocity (+Vmax), −2.047±0.30 vs −3.503±0.50 um/sec, Tx vs Eu, respectively, p<0.001; slower max relaxation velocity (‐Vmax), 1.369±0.27 vs 2.44±0.73 um/sec, p<0.01; longer time to peak contraction, 0.279±0.03 vs 0.182±0.03 sec, p<0.001. Myocytes were loaded with Fura2/AM (1uM) and intracellular calcium measured by fluorescence intensity ratio at 340/380 nm. Ca2+ transients showed slower kinetics in Tx vs Eu cardiomyocytes: time to Ca2+ peak, 0.076±0.01 vs 0.0516±0.003 sec, Tx vs Eu, respectively, p<0.0001; time to Ca2+ max return velocity, 0.261 0.03 vs 0.184 0.005, Tx vs Eu, respectively, p<0.0001. TTorg analysis indicated significant disorganization of T‐tubules in Tx cardiomyocytes compared to Eu (TT power = 61±2 vs 80±2, Tx vs Eu, respectively, p<0.00001; arbitrary units with higher values indicate increased periodicity). In conclusion, TH‐deficiency disrupts T‐tubule structures causing cardiomyocyte contractile dysfunction and slower calcium kinetics indicating altered EC‐coupling. Therefore maintaining normal thyroid function is necessary for optimal heart health.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.