Postnatal Development of T-Tubules in Sheep Atrial Myocytes

Abstract

Background: Cardiac t-tubules are essential in ensuring systolic calcium rises synchronously to enable contraction. The t-tubule network is absent or very sparse at birth in the ventricles of small mammals and develops postnatally with subsequent changes in calcium handling. Nothing is currently known about the development of t-tubules in the atria.Methods and Results: Newborn, 1 month, 3 month, 6 month and adult sheep (∼18 months) were euthanased and cells isolated from the left atrial appendage. T-tubule density assessed using di-4-ANEPPS and confocal microscopy showed partially developed t-tubules were present at birth in sheep atrial myocytes but at 30±9% of adult density (p<0.001). T-tubule density increased at 1 month and by 3 months t-tubules appeared fully developed despite decreased cell width compared to adult (p<0.01). In current clamp experiments, performed at 37°C with fluo-3 AM loaded cells, APD10, 25 and 50 were shorter in newborn, 1 month and 3 months compared to adult (p<0.01). There was no difference in calcium transient amplitude between newborn, 1 month and 3 month cells but the rate of decay of systolic calcium was slower in newborn compared to both 1 month and 3 months (p<0.05).Conclusion: T-tubules are present at birth in sheep atrial myocytes and develop over the first 3 months of life. Both action potential duration and rate of calcium transient decay increase during development. We hypothesise that this may be due to increased L-type calcium current as t-tubules develop and increased SERCA function as the sarcoplasmic reticulum matures. Further investigation is required to determine how the expression of t-tubule regulatory proteins and activity of calcium handling proteins changes throughout development.