Ca²⁺ protection from the negative inotropic effect of contraction frequency on teleost hearts

Summary

Isometric tension development by ventricular strips of 9 species of teleosts, a frog and a turtle was assessed at varying contraction frequencies and Ca_o (external calcium concentration). With teleost hearts an increase in contraction frequency at constant Ca_o was always associated with a decrease in tension development; however, under comparable conditions a positive staircase was exhibited by the frog and turtle heart preparations. The reaction of the teleost heart was thus very different from the well established response of the hearts of higher vertebrates. Elevations in Ca_o always resulted in an increase in tension development such that the positive inotropic effect of Ca_o could compensate for the negative effect of a high contraction frequency.

Perfused isolated cod hearts exhibited an increase in cardiac output and pressure development as a result of increases in Ca_o. At 30 contractions min⁻¹ a transition from 1–2 mM Ca_o led to a 68% increase in performance defined as the product of cardiac output times pressure development. The response was in excess of that of ventricular strips. At low Ca_o increases in rate from in situ resting levels to the high end of the physiological range resulted in a decrease in performance. Increases in Ca_o were able to ameliorate the detrimental effect of high imposed contraction frequency.

In conclusion, both ventricular strip and perfused heart experiments show that a positive inotropic effect of increased Ca_o can compensate for or even surpass the negative effect of high contraction frequency when both variables are at physiological levels. This finding could have relevance to the maintenance of cardiac performance during/or following intense swimming when both heart rate and plasma calcium may be elevated.