OBJECTIVE We have studied the mechanisms underlying the positive inotropic action of prostaglandin F2 alpha (PGF2 alpha) by monitoring intracellular calcium transients, intracellular pH, L-type calcium currents and cell shortening in isolated ventricular myocytes. METHODS Rat myocytes were loaded with fura-2AM for intracellular calcium measurements, or BCECF-AM for pH measurements. Cell shortening was recorded using an edge detection system, and L-type calcium currents measured using whole cell patch clamping. RESULTS PGF2 alpha (3 nmol l-1-3 mumol l-1 increased single myocyte shortening and reduced resting cell length in a concentration-dependent manner. While myocyte shortening was increased by PGF2 alpha, this was not associated with any change in the amplitude of intracellular calcium transients, diastolic calcium, or L-type calcium currents. However, the same myocytes were capable of responding to catecholamines with increases in calcium transient amplitude and L-type calcium currents. PGF2 alpha (3 mumol l-1 caused a reversible rise in intracellular pH of 0.08 +/- 0.01 pH units (n = 5, p < 0.05). The Na(+)-H+ exchanger inhibitor, HOE 694 (10 mumol l-1, abolished the PGF2 alpha-induced rise in pH and the increase in cell shortening. PGF2 alpha-induced increases in cell shortening and intracellular pH were also attenuated by the protein kinase C (PKC) inhibitor, chelerythrine (2 mumol l-1. CONCLUSIONS The positive inotropic action of PGF2 alpha appears to be mediated via activation of the Na(+)-H+ exchanger with the possible involvement of PKC. This suggests that PGF2 alpha-produces intracellular alkalosis, which then sensitizes cardiac myofilaments to calcium.