SwissPhysio - Abstract

Clinical studies have reported successful impulse conduction between donor and recipient cardiac tissue after heart transplantation. This observation raises the hypothesis that cardiac connective tissue forming at the sites of sutures might transmit electrical activation between donor and recipient cardiac tissue. This question was studied by investigating electrical impulse propagation across geometrically defined regions of cardiac fibroblasts intercalated into strands of cardiomyocytes.
Experiments were performed with 4 day old primary cell cultures of neonatal rat ventricular cardiomyocytes. Using patterned growth techniques, the cardiomyocytes were induced to form 10 mm long and 80 um wide strands which were interrupted by cardiac fibroblast inserts of variable length (<100 um; 100-200 um; >200 um). Preparations were stimulated at 2 Hz and impulse propagation across inserts was investigated by multiple site optical recording of transmembrane voltage (MSORTV) using the voltage sensitive dye di-8-ANEPPS. Experiments were performed at 36 centigrade. The exact dimensions of the fibroblast inserts were assessed by immunocytochemistry using antibodies against myomesin to identify the cardiomyocytes flanking the inserts.
As determined by microscopic observations of synchronous contractile activity, fibroblasts established successful conduction between cardiomyocytes as soon as 8 hours after being seeded into the inserts. MSORTV revealed length-dependent local propagation delays which amounted to 22+/-8 ms (insert length: 73+/-16 um, n=11), 36+/-15 ms (insert length: 146+/-22 um, n=17), and 54+/-16 ms (insert length: 241+/-35 um, n=6). Even though fibroblasts showed membrane potential changes typical for electrotonic current flow, it could not be ruled out that, in addition to gap junctional coupling, mechanical stretch relayed by the fibroblasts was involved in successful propagation as well. Therefore, additional experiments were performed where wildtype HeLa cells devoid of gap junctions were used instead of fibroblasts. In all of these experiments, impulse propagation across inserts (77–400 um wide; n=45) invariably failed, thus establishing that gap junctional coupling between fibroblasts and cardiomyocytes was essential for successful impulse conduction.
These findings illustrate that fibroblasts of cardiac origin are capable of transmitting electrical activation in cardiac tissue over appreciable distances. Such impulse propagation across the cellular constituents of connective tissue might contribute to the explanation of the previous clinical observation of impulse propagation from recipient to donor heart after transplantation.