Lactic acid accumulation is generally believed to be involved in muscle fatigue. However, one study reported that in rat soleus muscle (in vitro), with force depressed by high external K + concentrations a subsequent incubation with lactic acid restores force and thereby protects against fatigue. However, incubation with 20 mM lactic acid reduces the pH gradient across the sarcolemma, whereas the gradient is increased during muscle activity. Furthermore, unlike active muscle the Na + -K + pump is not activated. We therefore hypothesized that lactic acid does not protect against fatigue in active muscle. Three incubation solutions were used: 20 mM Na-lactate (which acidifies internal pH), 12 mM Na-lactate +8 mM lactic acid (which mimics the pH changes during muscle activity), and 20 mM lactic acid (which acidifies external pH more than internal pH). All three solutions improved force in K + -depressed rat soleus muscle. The pH regulation associated with lactate incubation accelerated the Na + -K + pump. To study whether the protective effect of lactate/lactic acid is a general mechanism, we stimulated muscles to fatigue with and without pre-incubation. None of the incubation solutions improved force development in repetitively stimulated muscle (Na-lactate had a negative effect). It is concluded that although lactate/lactic acid incubation regains force in K + -depressed resting muscle, a similar incubation has no or a negative effect on force development in active muscle. It is suggested that the difference between the two situations is that lactate/lactic acid removes the negative consequences of an unusual large depolarization in the K + -treated passive muscle, whereas the depolarization is less pronounced in active muscle.

Murine leukotriene B₄ (LTB₄) receptor (mBLT1) cDNA was identified by searching the EST database using human LTB₄ receptor as the query sequence. Expression of functional mBLT1 after injection of in vitro transcribed cRNA into Xenopus laevis oocytes was demonstrated as LTB₄-evoked, Ca²⁺-activated Cl^- currents recorded by two-electrode voltage clamp. From mBLT1-expressing oocytes, a dose-dependent relationship between the Ca²⁺-activated Cl^- current and LTB₄ concentration was demonstrated with an apparent EC₅₀ of 6.7 nM. Following LTB₄ stimulation of mBLT1, we observed two transient, spatially distinct Ca²⁺-activated, inwardly directed Cl^- currents in the oocytes: a fast peak current requiring relatively high LTB₄ concentrations, and a slowly progressing Cl^- current. Nucleotides, PGE₂, 12R-hydroxy-5, 8, 14-cis-10-trans-eicosatetraenoic acid, and LTD₄ did not activate mBLT1. U75302, specifically targeting BLT1, significantly reduced LTB₄-evoked Cl^- currents. Repetitive LTB₄ administration desensitized the LTB₄-evoked currents. Activation of protein kinase C (PKC) by PMA addition completely eliminated the LTB₄-evoked currents, whereas down-regulation of PKC by prolonged PMA exposure (20 h) impaired mBLT1 desensitisation. In addition, Ser-127-Ala substitution of the PKC consensus phosphorylation site on the second intracellular loop prevented the mBLT1 desensitisation. These data indicate that PKC-mediated phosphorylation at Ser-127 leads to mBLT1 desensitisation.