Impaired sarcolemmal vesicle lactate uptake and skeletal muscle MCT1 and MCT4 expression in obese Zucker rats

The present experiments were undertaken to characterize 1) the hindlimb muscle mass lactate uptake and 2) the expression of monocarboxylate transporter isoforms MCT1 and MCT4, as well as lactate dehydrogenase (LDH) isozyme distribution, in various skeletal muscles of Zucker fa/fa rats taken as a model of insulin resistance-related obesity. Initial lactate uptake at six different concentrations was measured in sarcolemmal vesicles (SV) by use ofl-[U-14C]lactate. Compared with controls, the maximal rate of lactate uptake and affinity were decreased in SV of Zucker rats (∼30%) in which MCT4 content was significantly decreased ( P < 0.05). MCT4 expression was decreased in soleus, extensor digitorum longus, and red tibialis anterior (RTA; P < 0.05), but not in white tibialis anterior, whereas MCT1 expression was decreased only in RTA of Zucker rats ( P < 0.05). Obesity led to a shift toward type M-LDH isozyme in mixed muscles. We conclude that obesity leads to changes in muscular MCT1 and MCT4 expression, which, when associated with LDH isozyme redistribution, may contribute to the hyperlactatemia noted in insulin resistance.

On the Muscle Fibres and Locomotor Activity of Doliolids (Tunicata: Thaliacea) Q. Bone

Doliolids are pelagic tunicates which swim by very rapid contractions of the muscle bands that encircle their barrel-shaped bodies; the resulting jet pulses drive them forwards at instantaneous velocities up to 50 body lengths s-1 (Bone & Trueman, 1984). The obliquely-striated muscle fibres within the bands have the unique feature that they lack any kind of sarcoplasmic reticulum or sub-sarcolemmal vesicle system (Bone & Ryan, 1974). Most stages of the rather complex life-cycle of doliolids (Braconnot, 1971a) are small animals, less than 4 mm long; but in one of the species examined the largest may be up to 40 mm. This paper shows that external Ca2+ is required for contraction of the locomotor muscle fibres and that the decremental muscle potentials preceding contractions are carried by Ca2+. In younger stages, the muscle fibres within a band are electrically coupled to some extent, but in older animals, the degree of coupling decreases