Decreased intercellular coupling after prolonged rapid stimulation in rabbit atrial muscle

Driving rabbit atrial trabeculae at a rapid rate for 15 minutes resulted in a decrease in the space constant for electrotonic decay from an average of 670 to 400 μm. Input resistance, R, as measured by use of a double-barrelled microelectrode, increased from a mean value of 380 kOhms to one of 600 kOhms. The time to return to control values after the end of rapid driving was 20-60 minutes. Similar effects of rapid driving were observed in the presence of atropine, propranolol, and atropine plus propranolol and phentolamine. According to the theory of current spread in a three-dimensional syncytium, a rise of input resistance should be interpreted mainly as an increase of cell-to-cell resistance. We advance the hypothesis that, when driven at their maximal possible rate (or when fibrillating), cardiac cells gain Na⁺ and Ca²⁺, and that this results in partial but reversible uncoupling.