Cytokines including tumour necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β) are known to mediate systolic and diastolic myocardial dysfunction in inflammatory disease such as sepsis. To provide a cellular basis, we separately studied the effects of 50 ng/ml TNF-α and IL-1β on intracellular Ca handling and contractility in sheep ventricular myocytes.
All procedures used accord with the Animals (Scientific Procedures) Act, UK, 1986 and Directive 2010/63/EU of the European Parliament. Ventricular myocytes were isolated from young (aged 18 months) sheep, and loaded with the ratiometric Ca indicator Fura-2. Cells were field stimulated at 0.5 Hz and intracellular Ca and contractility dynamics measured by epi-fluorescent photometry and video sarcomere detection respectively. Relative changes in SR Ca content were estimated from the amplitude of Ca transients evoked by application of 10 mM caffeine.
50 ng/ml TNF-α and IL-1β reduced SR Ca content by 27% and 41% respectively, accounting for a 17% and 24% reduction each in the amplitude of systolic Ca. With TNF-α, the reduction of systolic Ca was associated with a 20% reduction in sarcomere shortening, however IL-1β increased sarcomere shortening. The rate constant of systolic Ca decay was unaffected by both TNF-α and IL-1β. In response to both TNF-α and IL-1β the onset of systolic Ca decrease was rapid (<10 s), however, in 71% of TNF-α treated cells and 52% IL-1β treated cells, this was preceded by an immediate increase (58% and 52% respectively) in systolic Ca, lasting for only 1–3 beats. TNF-α decreased diastolic Ca by 4%, which was unchanged by IL-1β whilst diastolic sarcomere length was decreased by both cytokines.
Both TNF-α and IL-1β reduced SR Ca content accounting for reduced systolic Ca. In the case of TNF-α, this was associated with reduced contractility whereas with IL-1β, paradoxically, contractility was increased. This and the fact that resting sarcomere length was decreased despite no increase in diastolic Ca suggest both cytokines may increase myofilament sensitivity. The mechanism by which SR Ca content is reduced remains unclear. Whilst SERCA impairment does not appear to play a role, the initial and short-lived increase of systolic Ca may suggest a role for ryanodine receptor potentiation; a phenomenon we are currently investigating. While the changes to cell function produced by both cytokines can account for certain aspects of myocardial depression in sepsis, their individual contribution to the clinical phenotype may be more complex than previously thought.