Physical, contractile and calcium handling properties of neonatal cardiac myocytes cultured on different matrices

Extracellular matrix components play a vital role in the determination of h eart cell growth, development of spontaneous contractile activity and morph ologic differentiation. In this work we studied the physical and contractil e changes in neonatal rat cardiac myocytes over the first four days of grow th on three different extracellular matrices. We compared commercial lamini n and fibronectin, plus a fibroblast-derived extracellular matrix, which we have termed cardiogel. Myocytes cultured on cardiogel were characterized b y greater cellular area and volume when compared to cells cultured on the o ther single-component matrices. Spontaneous contractile activity appeared f irst in the cells grown on cardiogel, sometimes as early as the first day p ost-plating, in contrast to day three in the cells cultured on laminin. Mea surements of cardiac myocyte contractility i.e. percent shortening and time to peak contraction, were made on each of the first four days in each cult ure. Myocytes cultured on cardiogel developed maximum shortening more rapid ly than the other cultures, and an earlier response to electrical pacing. H istochemical staining for myocyte mitochondrial content, revealed that the cardiogel-supported cells exhibited the earliest development of this organe lle and, after four days, the greatest abundance. This reflects both a grea ter cell size, as well as response to increasing energy demands. Due to the increase in volume and contractile activity exhibited by the car diogel grown myocytes, we employed calcium binding and uptake experiments t o determine the comparative cellular capacities for calcium and as an indic ator of sarcoplasmic reticulum development. Also whole cell phosphorylation in the presence of low detergent was assayed, to correlate calcium uptake with phosphorylation, in an attempt to examine possible increases in calciu m pump number and other phosphorylatable proteins. In agreement with our ph ysical and contractile data, we found that the cells grown on cardiogel sho wed a greater calcium uptake over the first four days of culture, and incre ased phosphorylation. However, calcium binding was not dramatically differe nt comparing the three culture matrices. Based on our data, the fibroblast-derived cardiogel is the matrix of choice supporting earliest maturation of neonatal cardiomyocytes, in terms of spo ntaneous contractions, calcium handling efficiency, cell size and developme nt of a subcellular organelle, the mitochondrion.