EXPERIMENTS ON THE MECHANISM OF ACTION OF CHLOROCRESOL AND CAFFEINE

Synthetic localized calcium transients directly probe signalling mechanisms in skeletal muscle

The contribution of Ca2+-induced Ca2+ release (CICR) to trigger muscle contraction is controversial. It was studied on isolated muscle fibres using synthetic localized increases in Ca2+ concentration, SLICs, generated by two-photon photorelease from nitrodibenzofuran (NDBF)-EGTA just outside the permeabilized plasma membrane. SLICs provided a way to increase cytosolic [Ca2+] rapidly and reversibly, up to 8 μM, levels similar to those reached during physiological activity. They improve over previous paradigms in rate of rise, locality and reproducibility. Use of NDBF-EGTA allowed for the separate modification of resting [Ca2+], trigger [Ca2+] and resting [Mg2+]. In frog muscle, SLICs elicited propagated responses that had the characteristics of CICR. The threshold [Ca2+] for triggering a response was 0.5 μM or less. As this value is much lower than concentrations prevailing near channels during normal activity, the result supports participation of CICR in the physiological control of contraction in amphibian muscle. As SLICs were applied outside cells, the primary stimulus was Ca2+, rather than the radiation or subproducts of photorelease. Therefore the responses qualify as ‘classic' CICR. By contrast, mouse muscle fibres did not respond unless channel-opening drugs were present at substantial concentrations, an observation contrary to the physiological involvement of CICR in mammalian excitation–contraction coupling. In mouse muscle, the propagating wave had a substantially lower release flux, which together with a much higher threshold justified the absence of response when drugs were not present. The differences in flux and threshold may be ascribed to the absence of ryanodine receptor 3 (RyR3) isoforms in adult mammalian muscle.

Antileishmanial activity of sodium stibogluconate fractions

Sodium stibogluconate, a pentavalent antimony derivative produced by the reaction of stibonic and gluconic acids, is the drug of choice for the treatment of leishmaniasis. It has been reported to be a complex mixture rather than a single compound. We separated sodium stibogluconate into 12 fractions by anion-exchange chromatography. One fraction accounted for virtually all the leishmanicidal activity of the fractionated material against Leishmania panamensis promastigotes, with a 50% inhibitory concentration (IC50) of 12 micrograms of Sb per ml; that of unfractionated sodium stibogluconate was 154 micrograms of Sb per ml. Further analysis of this active fraction revealed that a major component was m-chlorocresol, which had been included in the sodium stibogluconate formulation as a preservative. The IC50 of pure m-chlorocresol was 1.6 micrograms/ml, a concentration equivalent to that present in unfractionated sodium stibogluconate at a concentration of 160 micrograms of Sb per ml. After ether extraction to remove m-chlorocresol, the IC50 of sodium stibogluconate was > 4,000 micrograms of Sb per ml. In contrast, when L. panamensis amastigotes were grown in macrophages, the IC50 of ether-extracted sodium stibogluconate was 10.3 micrograms of Sb per ml. The 12 fractions of ether-extracted sodium stibogluconate obtained by anion-exchange chromatography had IC50s of 10.1 to 15.4 micrograms of Sb per ml. We conclude that preservative-free sodium stibogluconate has little activity against L. panamensis promastigotes but is highly active against L. panamensis amastigotes in macrophages. This activity is associated with multiple chemical species.