- Addition of zatebradine, a direct sinus node inhibitor, provides no greater exercise tolerance benefit in patients with angina taking extended-release nifedipine: results of a multicenter, randomized, double-blind, placebo-controlled, parallel-group study. The Zatebradine Study Group. Frishman, W.H., Pepine, C.J., Weiss, R.J., Baiker, W.M. J. Am. Coll. Cardiol. (1995)