Effects of exogenously applied guanosine triphosphate on membrane current and tension of bullfrog atrial muscle

Abstract

The effects of exogenously applied guanosine triphosphate (GTP) were studied on bullfrog atrium by a double-gap method under voltage clamped and unclamped conditions. GTP produced an immediate augmentation of contraction showing its maximal effect within 5 to 10 min and a secondary gradual depression after longer perfusion. The positive inotropic effects of GTP were similar to those of adrenaline, although they were not prevented by the most effective dose of propranolol. Voltage clamp study revealed that GTP enhanced the slow calcium inward current ( I Ca) and I Ca-dependent phasic tension, while inhibited the I Ca-independent tonic tension. The delayed outward current was initially depressed and later augmented with time. The fast sodium inward current, the anomalous rectification and the background currents were virtually unchanged. GTP may act on the sarcolemmal level, at least on a site closely related to the movement of calcium ions through the slow channel, and on one or more unrelated sites. The exact mechanism of these effects is uncertain, but GTP exerts a positive inotropic effect concommitant with an increase in the slow inward current which was not prevented by the β-blocking agent.