Chronotropic response of spontaneously beating rabbit atria to hyperosmotic media

Spontaneously beating rabbit atria responded to hyperosmotic Tyrode bathing media with an increase in rate, force of contraction, and passive tension dependent on the level of osmolality and the osmotic agent employed. The positive chronotropic response reached a maximum within a few minutes and then declined to a lower, maintained plateau level. The plateau change in rate was similar whether the osmotic agent added to Tyrode solution was sucrose, mannitol or NaCl. For these agents, the response increased linearly with osmotic pressure of the bathing media from 300 to 500 mosmol/kg H2O, then progressively decreased approaching zero (plateau rate = control) at about 700 mosmol/kg H2O. The chronotropic response to urea in Tyrode solution, though less than for the other three agents, progressively increased over the entire range of osmolalities tested (from 300 to 700 mosmol/kg H2O). The inotropic response was positive for all agents from 300 to 600 mosmol/kg H2O. Passive tension of atria increased with added NaCl, sucrose, or mannitol, but not with urea. Propranolol did not alter the atrial response to hyperosmolality. Thus, we find that the chronotropic response of atria to hyperosmolar solutions is positive over a wide range of agents and osmolalities, in contrast to earlier reports of a direct negative chronotropic effect.