DEPRESSED TRANSIENT OUTWARD AND CALCIUM CURRENTS IN DILATED HUMAN ATRIA

Objective: The aim was to compare action potentials and ionic currents (steady state current, calcium current, calcium independent transient outward current) in two groups of trabeculae and myocytes, isolated f rom either dilated or non-dilated human atria. Methods: Specimens of r ight atrial appendage were obtained from two groups of adult patients at the time of open heart surgery, a group with non-dilated atria and a group in which right atria were clearly dilated. Action potentials w ere recorded with standard microelectrodes from isolated superfused tr abeculae. Action potentials and ionic currents were recorded from sing le myocytes using the patch clamp technique in the whole cell configur ation in current clamp and voltage clamp modes respectively. Results: In trabeculae taken from dilated atria the action potential was shorte ned and the plateau was markedly depressed compared to trabeculae take n from non-dilated atria. Similar results were obtained with single my ocytes isolated from non-dilated and dilated atria. The density of the steady state current measured at the end of 0.75 s or 1 s pulses was not statistically different in the two groups of cells in the whole ra nge of negative potentials, whereas at strongly positive potentials (> +40 mV) it was significantly reduced in cells from dilated atria compa red to cells from non-dilated atria. The density of the total peak out ward current was significantly reduced in cells from dilated atria [13 .46(SEM 2.7) pA.pF(-1) at +70 mV, n = 18] compared to cells from non-d ilated atria [33.12(6.2) pA.pF(-1), n = 20, p < 0.001]. The transient component of outward current was strongly depressed (at +20 mV and mor e positive potentials) in cells from dilated atria. The calcium curren t density was still more severely depressed than the total outward cur rent in cells from dilated atria [4.46(1.06) pA.pF(-1) at +20 mV, n = 26] compared to cells from non-dilated atria [17.43(1.98) pA.pF(-1), n = 38, p << 0.001]. Kinetic parameters of both calcium and transient o utward currents remained similar in cells from the two groups. Conclus ions: The observation that in cells from dilated human atria the calci um current is more severely depressed than the total outward current c an help to explain why in dilated human atria the action potential pla teau is shorter and of lower amplitude than in non-dilated atria.