Ionic mechanisms underlying the enhancement of cardiac pacemaking activity by mechanical stretch were investigated in the isolated rabbit sinoatrial (SA) node. A 5-s stretch of 0.2-2.0 g was applied to small tissue strips (1.5 mm x 3.0 mm) of the SA node using a mechanical stimulator. Spontaneous excitation cycle length (SPCL) was monitored by recording endocardial surface potential through modified bipolar electrodes with high-gain amplification. Influence of neurotransmitters released from nerve terminals was eliminated by atropine and propranolol. A stretch > 0.2 g caused a significant shortening of SPCL; there was a positive correlation between the force and the maximum shortening of SPCL. Treatment of the preparation with gadolinium (10 microM) or glibenclamide (1 microM) did not affect the force-response relationship. The positive chronotropic response to mechanical stretch > 0.5 g was reduced significantly by treatment with 4,4'-dinitrostilbene-2,2'-disulfonic acid (5mM), 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (1 mM), or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (50 microM). The positive chronotropic response was also reduced in a low-Ca2+ (0.36 mM) medium and by bath application of ryanodine (0.1 microM) or thapsigargin (2 microM). These findings suggest the possible involvement of mechanosensitive Cl- channels and intracellular Ca2+ mobilization in the stretch-induced enhancement of pacemaking activity in the mammalian SA node, although other conceivable mechanisms cannot be ruled out.