The intraocular pressure lowering effect of colchicine

A mechanism for trabecular meshwork cell retraction: ethacrynic acid initiates the dephosphorylation of focal adhesion proteins

Ethacrynic acid (ECA) increases aqueous humor outflow facility in human and animal model systems, and causes cellular retraction in cultured trabecular meshwork (TM) cells. ECA-induced retraction, a possible correlate to the opening of spaces in the outflow pathway in vivo, takes place coincident with disruption of cell-cell attachments and actin stress fibers. Tyrosine phosphorylated proteins are located predominantly where actin filaments terminate at sites of cell-to-cell and cell-to-substrate adhesion, and are understood to regulate cellular adhesions and filamentous (F) actin organization in many cell types. In the present study we investigated whether ECA might affect cell adhesions and F-actin in TM cells by altering levels of phosphotyrosine. We analysed levels of phosphotyrosine in cultured human TM and calf pulmonary artery endothelial cells after exposure to ECA. Using immunoflourescence microscopy and antibodies to phosphotyrosinated proteins we found a rapid decrease in phosphotyrosine levels at the focal contacts of cells treated with ECA. Immunoblots of whole cell extracts showed a decrease in phosphotyrosine predominantly in a band running at about 120 kD, with a more subtle decrease in a band about 65 kD. Reprobing the blot with antibodies to pp120 focal adhesion kinase (FAK) or paxillin indicated that the 120 kD band was FAK and the 65 kD band was likely paxillin. Immunoprecipitation of FAK or paxillin and probing the resulting blot with antibodies to phosphotyrosine confirmed that these proteins were rapidly dephosphorylated after ECA addition. Loss of FAK and paxillin proteins in cells was then confirmed using immunofluorescence microscopy. Dephosphorylation of these proteins was detected before the onset of retraction, stress fiber disruption, or complete disruption of focal adhesions. A pure microtubule inhibitor (colchicine), did not cause stress fiber disruption or decrease focal adhesion phosphorylation. We postulate that dephosphorylation of FAK and paxillin by ECA disrupts signaling pathways that normally maintain the stability of the actin cytoskeleton and cellular adhesions, and that this action leads both to cell shape change in culture, and to facility changes in vivo.

Inhibition of the acute ocular responses to nitrogen mustard by colchicine

Colchicine, a naturally occurring alkaloid, inhibits both fast and slow axoplasmic transport by prevention of microtubule assembly. We have examined the influence of colchicine on the neurogenically mediated acute inflammatory responses to topically administered nitrogen mustard and formaldehyde on the albino rabbit eye. Topical administration of colchicine (10, 20 or 40 micrograms) once per day for 15 days inhibited the ocular hypertensive response, the leakage of protein into the anterior chamber, and the pupillary constriction observed following topical administration of nitrogen mustard. Colchicine pretreatment also inhibited the ocular hypertensive response to topically administered formaldehyde. Pretreatment of the eye with colchicine (20 micrograms day-1) for a period of 5 days proved to be ineffective in suppressing the ocular hypertensive effect of nitrogen mustard, while more prolonged pretreatment (10 or 15 days) significantly abrogated the characteristic injury responses. Topical administration of colchicine (20 and 40 micrograms) caused dilation of conjunctival and limbal vessels, but these vascular responses subsided as treatment continued. These preliminary experiments suggest that the inhibition of the ocular responses to nitrogen mustard by chronic colchicine pretreatment might be the result of modification of synthesis or release of sensory mediator(s).

The effect of colchicine on the rate of formation of aqueous humour and the gross outflow facility in the albino rabbit

Colchicine, a naturally occurring plant alkaloid which prevents the polymerisation of cytoplasmic microtubules, lowers the intraocular pressure after topical administration or intravitreal injection. In this study we have examined the effect of topically administered colchicine on the rate of formation of aqueous humour and the gross outflow facility in the albino rabbit eye. The disappearance of [14C]inulin from the anterior chamber was measured to calculate the rate of aqueous humour formation and in a separate group of animals the gross outflow facility determined using a constant pressure perfusion technique. Topically administered colchicine (10, 20 and 40 micrograms/eye) inhibited the rate of aqueous humour formation dose-dependently. The mean rate of formation in control eyes was 3.68 +/- 0.09 microliters/min (n = 16) decreasing to 1.8 microliter/min (n = 9) in eyes treated with 40 micrograms colchicine. Furthermore, the gross outflow facility of colchicine treated (10 micrograms/eye) eyes (0.49 +/- 0.03 microliter/min/mmHg) was significantly greater (P less than 0.05) than that of contralateral control eyes (0.39 +/- 0.03 microliter/min/mmHg). The pharmacological evidence available indicates that colchicine is acting via mechanisms of microtubule disruption to produce an ocular hypotensive response, suggesting that microtubules may be involved in the formation of aqueous humour and possibly the maintainance of cell shape and form in the outflow vessels of the anterior chamber.

Reduction in intraocular pressure induced by colchicine and related compounds

The effects of various microtubule inhibitors on the intraocular pressure (IOP) of albino rabbits were investigated. The compounds produced a marked fall in the IOP which was more prolonged after intravitreal injection than after topical administration. Tolerance to this response developed although no cross tolerance between colchicine and vinblastine was apparent. The drugs produced no effect on the pupil diameters and fluorescein angiography revealed no leakage of dye from the vessels of the iris. Lumicolchicine and trimethylcolchicinic acid were without effect on the IOP. The iris-ciliary processes contained more [3H]colchicine after intravitreal injection than after topical administration, and most was associated with the ciliary processes, the site of formation of aqueous humour. Further colchicine and vinblastine were bound within the tubulin containing fraction of this tissue. We suggest that colchicine and related compounds reduce the IOP via mechanisms of microtubule disruption which may partially inhibit the formation of aqueous humour by the ciliary processes.