16 Does Inorganic Phosphate inhibit Cardiac Myofibrillar Force Production at 37°C?

In the healthy heart, inorganic phosphate (Pi) is present in millimolar concentrations, rising to 20 millimolar or more in ischaemic heart. Since acute myocardial ischaemia leads to heart failure, there is need to gain better functional understanding of cardiac preparations in the presence of added Pi. Studies with skinned cardiac myocytes over the range 15°C–23°C have shown that Pi suppresses myofibrillar force generation. However, in skinned skeletal fibres a similar inhibitory action of Pi at low temperatures is reduced or abolished at temperatures closer to physiological. We therefore compared the effects of Pi on force development in skinned single cardiomyocytes at 15°C and 37°C. Permeabilised single cardiomyocytes from human ventricular myocardium (either from healthy, unused donor or from failing hearts) were maximally activated (30 μM free Ca2+) in the presence of Pi (0–20 mM added). A rapid 1% stretch was applied during isometric force development to additionally measure crossbridge kinetics. Maximum Ca2+ activated force decreased exponentially as [Pi] was raised. The sensitivity to Pi was similar for the two temperatures, e.g. 20 mM added Pi decreased force by ~87% at 15°C and ~78% at 37°C (n = 4, P = 0.74). The rates of stretch-induced cross-bridge detachment (krel) and the subsequent force re-development (kdf) increased exponentially as a function of Pi. These findings suggest that the inhibitory effect of Pi on myofibrillar force development is substantial at both 15°C and 37°C. This deleterious effect is therefore likely to contribute to the contractile dysfunction induced by cardiac ischaemia in vivo.