Mechanisms for hypothermia-induced increase in contractile force studied by mechanical restitution and post-rest contractions in guinea-pig papillary muscle

Lowering myocardial temperature increases contractile force, presumably by increasing intracellular calcium content. To study the mechanisms behind this, we compared the effects of some known inotropic interventions with hypothermia on mechanical restitution and post-rest contractile force in isolated guinea-pig papillary muscles. In four groups (n = 6 per group), the effects of: (1) reducing the ability for Na/Ca exchange to extrude Ca²⁺ (a) by increasing [Na⁺]₁ with ouabain or (b) by increasing [Ca²⁺]_o; and (2) activation of calcium channels with Bay-K 8644, were compared with lowering temperature from 37 to 27 °C. Normally (at 37 °C and 2 mm CaCl₂), mechanical restitution could be described by a rapid recovery phase with a time constant between 180 and 220 ms, followed by a slowly decaying phase with a time constant between 5000 and 8000 ms and post-rest contractions (1–10 min rest) were markedly depressed compared to steady-state contractions. Steady-state developed force was markedly increased at 27 °C, after 1 μm ouabain, 6 mm CaCl₂ or 0.1 μM Bay-K 8644. At 27 °C the rapid recovery phase of restitution was delayed while the slowly decaying phase was not affected. Ouabain and increased [Ca²⁺]_o caused elevation of the slowly decaying phase of restitution and markedly attenuated the post-rest depression of developed force, which may be attributed to a reduced diastolic extrusion of Ca²⁺ via the Na/Ca exchanger. Hypothermia and Bay-K 8644 on the other hand, augmented this post-rest depression. Hence, this study suggests that increased Ca²⁺ influx due to delayed inactivation of calcium channels may account for the increased developed force during hypothermia rather than reduced diastolic extrusion of Ca²⁺ via the Na/Ca exchanger.