Inorganic Phosphate Enhances Systolic and Diastolic Function in Excitable Papillary Muscle and Accelerates Myofilament Relaxation in Response to Lengthening

Abstract

Inorganic phosphate (Pi) is an essential constituent of blood serum found in concentrations of roughly 1.1-1.4 mM. Cardiac myofilament force is reduced with increasing Pi due to its facilitating reversal of the force-producing myosin power stroke. This Pi-dependent reversal of the power stroke is observed to be further augmented by lengthening in demembranated myocardium. We hypothesized that relaxation in excitable myocardium is enhanced by increasing extracellular Pi concentrations and especially when the myocardium is lengthened during the relaxation phase of the force transient. We tested this hypothesis using excitable myocardium isolated from female Wistar rats aged 4-5 months. Rats were fully anesthetized, and their hearts removed. Left ventricular papillary muscles were attached to platinum clips, placed between a force transducer and a length motor at room temperature, and bathed in Krebs solution with 1.8 mM Ca2+ and varying Pi of 0, 1, 2 and 5 mM. Force transients were elicited by electrical stimulation at 1 Hz. Peak tension was statistically unchanged with Pi: 0.641±0.085 mN.mm-2 at 0 mM Pi and 0.749±0.153 mN.mm-2 at 5 mM Pi (mean±SEM, n=9, p=0.46 for repeated measures ANOVA). The maximum rate of tension development (+dP/dt max) normalized to peak tension was significantly enhanced by Pi: 11.0±0.3 s-1 at 0 mM Pi and 13.4±0.5 s-1 at 5 mM Pi (p<0.05). All temporal characteristics of the force transient were significantly shortened with increasing Pi. Time-to-peak was shortened from 162±6 ms at 0 mM Pi to 135±4 ms at 5 mM Pi (p<0.01), and time-to-50% recovery was shortened from 309±12 ms at 0 mM Pi to 258±9 ms at 5 mM Pi (p<0.01). A 1% lengthening stretch with varying duration of 10-200 ms was applied at time-to-50% recovery during the descending phase of the force transient. The recorded force response represented the full viscoelastic response of the activated muscle in late systole/early diastole. Matching lengthening stretches were also applied when the muscle was not stimulated, thus providing a control for the passive viscoelastic response. After subtracting the passive from the active force response, the resulting myofilament response demonstrated features of faster myofilament relaxation in response to the stretch. Temporal characteristics of the resulting myofilament response to stretch were significantly shortened due to the applied lengthening and with increasing Pi. For example, time-to-70% relaxation with 100 ms lengthening duration was shortened by 8.8±6.8 ms at 0 Pi, 19.6±4.8* ms at 1 mM Pi, 31.0±5.6* ms at 2 Pi, and 25.6±5.3* ms at 5 mM Pi (*p<0.01 compared to no change). Our findings suggest that increasing Pi availability at the myofilaments contributes directly to myofilament relaxation due to reversal of the myosin power stroke and especially as the myocardium transitions to relengthening in diastole.