Biphasic contractile response to ouabain: Species specific? Calcium dependent? Altered sensitivity?

Summary

Up to now, only the rat, an “ouabain insensitive” rodent, has shown a consistent biphasic positive inotropic response to ouabain, “low-dose” effect ED₅₀ ~ 0.3–0.5 µM, “high dose” effect ED₅₀ > 10 µM. The “low-dose” (high affinity?) effect was masked by high Ca⁺⁺ (2 mM) and emphasized in low Ca⁺⁺ (0.5 mM). The “low-dose” inotropic effect in rat ventricular strips was significantly diminished by repeated exposure to high ouabain concentrations as was ³H-ouabain binding (K_D and B_max). The decrease of the “low-dose” effect could be desensitization, tachyphylaxis (quick tolerance) or continued receptor occupancy by residual ouabain. The contractile activity of the preexposed strips did not indicate the presence of residual ouabain since their basal contractile force was decreased 12% compared to initial control instead of an expected 20% increase if ouabain occupancy had continued. The maximal response to high ouabain concentrations was identical in both exposures. In Langendorff perfused rat heart preparations the “low-dose” response was not diminished by repeated ouabain exposure and ³H-ouabain binding was not altered. The “low-dose” effect was not present in rat atrial strip preparations.

Although the I₅₀ of rat Na⁺,K⁺-ATPase (NKA) inhibition by ouabain is very high (~ 50 µM) and the low dose effect occurred with an ED₅₀ of 0.3–0.5 µM, the low dose effect was nevertheless related to inhibition of NKA since it was accompanied by an increase in Na, activity (12, 16). The minimal effect of Na_i was observed at < 0.1 µM ouabain.

Up to now, no other species has consistently shown a biphasic positive inotropic response to ouabain, although episodic observations have been reported (15). In rabbit, cat and guinea-pig Langendorff hearts, and atrial and ventricular strip preparations only monophasic, not biphasic dose-response curves (10⁻¹¹ to 10⁻⁶ M) could be obtained. Low calcium (0.8 mM) did not bring about an appearance of the lacking “low-dose” response in these “glycoside-sensitive” species. In guinea-pig, rabbit and cat hearts (9) no biphasic or other “low-dose” effects were produced by alteration of temperature (30–37°C); stimulation rate (14 Hz); time (immediate exposure or 2 to 4 hour equilibration with possible loss of external Ca pools); alterations of on concentrations of Mg (0.1 to 1.2 mM), K⁺ (2.7 to 9 mM) or Ca⁺⁺ (0.5–2.5 mM); and repetitive exposure to ouabain.