Inhibition of sarcoplasmic reticulum function by polyunsaturated fatty acids in intact, isolated myocytes from rat ventricular muscle

Polyunsaturated fatty acids (PUFAs) present in fish oils are known to have protective effects against arrhythmias generated post-infarction and when present in the diet, seem to protect against heart disease in general (for review see Leaf et al. 1999). Much work has been carried out on the cardiac effects of PUFAs and how protection is effected. PUFAs such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) inhibit sodium and L-type calcium currents in cardiac myocytes (Xiao et al. 1997; Macleod et al. 1998). It has also been shown that the delayed rectifier (Honore et al. 1994) and transient outward currents (Macleod et al. 1998) are inhibited. The net effect of these changes in surface membrane currents is depressed electrical excitability (Kang et al. 1995), clearly this will help to reduce the occurrence of arrhythmias and can explain the protective effect. An important cause of cardiac arrhythmias is abnormal functioning of the sarcoplasmic reticulum (SR). Thus far, however, relatively little work has examined possible effects of PUFAs on cardiac sarcoplasmic reticulum function. Post-ischaemia damaged cardiac myocytes undergo delayed afterdepolarisations that may be large enough to generate an arrhythmogenic action potential (Stern et al. 1988). The cause of the depolarisation is spontaneous release of calcium from the sarcoplasmic reticulum (Kass et al. 1978). Following ischaemic damage, cellular calcium regulation is compromised and the SR becomes overloaded with calcium and prone to spontaneous release events. This spontaneous release takes the form of propagating waves of calcium-induced calcium release (CICR) and has been demonstrated both in single, isolated cardiac myocytes and in multicellular preparations (Wier et al. 1987; Daniels et al. 1991). The raised intracellular calcium concentration during propagating waves activates the Na+-Ca2+ exchanger and an inward, depolarising current is generated (Lipp et al. 1987, 1990). If large enough, the waves can generate sufficient inward current and depolarisation to initiate an action potential. Although depression of surface membrane excitability may make it more difficult for the wave to cause an action potential, it is less likely to be of great importance to generation of waves. It is known, however, that modulation of SR function can affect the frequency and amplitude of propagating waves; both important determinants of the likelihood of generating arrhythmias e.g. inhibition of calcium release from the SR leads to less frequent but larger waves (Overend et al. 1997). Some evidence that PUFAs affect SR function has recently been reported (Rodrigo et al. 1999) in chemically skinned ventricular myocytes. These authors suggested that SR calcium release is inhibited by PUFAs but could not rule out inhibition of calcium uptake. The purpose of the present study was to determine what, if any, effect PUFAs have on arrhythmogenic, propagating waves of calcium-induced calcium release in intact, ventricular myocytes and whether this may provide additional protection against arrhythmias. We have studied the actions of EPA on electrically stimulated contractions and spontaneous waves in single cardiac myocytes isolated from rat ventricular muscle. Our results indicate that PUFAs do indeed affect SR function in two ways; through inhibition of calcium release and reduced availability of calcium. Therefore, at least part of the anti-arrhythmic action of PUFAs must be at the level of the SR.