Biomedical subjects

Vincent Lam

Publications and source records attributed to Vincent Lam.

4 recordsLinked to original sources

PubMed2005

Causation and space-time.

This paper considers the physical accounts of causation in terms of conserved quantities in the light of the theory of general relativity. As it is rather well-known among physicists, there are several difficulties with the notions of conservation and localization of the (gravitational) energy-momentum within general relativity. We first begin to review the so-called conserved quantity theory of causation mainly due to Dowe and Salmon, then we discuss some consequences of these difficulties for this physical account of causation. We argue that these difficulties are due to the fundamental nature of the space-time structure as described by GR, which the conserved quantity theory of causation does not account for.

Causality↗

PubMed2004

Pharmacokinetics of insulin uptake by ocular tissues and the role of cerebrospinal fluid in optic nerve insulin accumulation following topical insulin application.

BACKGROUND: As part of our ongoing studies concerning the efficacy of using topically applied medications to treat retinal and optic nerve diseases, we previously showed that insulin accumulated in the retina, optic nerve, and cerebrospinal fluid (CSF) following topical application. The purpose of this study was to investigate which route insulin takes to get to the posterior segment of the eye, and to specifically assess the role of the CSF in optic nerve insulin accumulation. METHODS: Lewis rats that received 125I-insulin eye drops were killed at different time points and their ocular tissues counted in a gamma counter. In order to determine whether elevated levels of CSF insulin could lead to optic nerve insulin accumulation, a separate cohort of animals was injected in their lumbar cistern with unlabeled insulin and their optic nerves later assessed for the presence of insulin by enzyme-linked immunosorbent assay. RESULTS: All ocular tissues (except the lens) showed at least one significant time point elevation in 125I-insulin compared to baseline. Both the combined cornea/iris/ciliary body and the sclera showed a notable, overall increase in counts over baseline, with values at 20 and 30 minutes being significantly elevated. The highest numbers of counts were seen in the aqueous humor. Animals injected intralumbar cisternally with insulin showed elevated insulin concentrations in their optic nerves and cisterna magna-derived CSF that did not appear to be due to uptake of insulin from the circulation. CONCLUSIONS: These results support the hypothesis that the insulin that accumulates in the retina and optic nerve following topical application arrives there after diffusing through the sclera, though an intraocular route--while unlikely--cannot be ruled out.

Administration, Topical↗

PubMed2003

Accumulation of porcine insulin in the rat brain and cerebrospinal fluid following ocular application.

We previously reported that insulin accumulated in the retina and optic nerve following ocular application. Since the optic nerve is surrounded by meninges and cerebrospinal fluid (CSF) and since it extends back to the thalamus, we examined whether the topical application of insulin eye drops also resulted in the accumulation of insulin in the CSF and brain. The data presented in this paper show that this is in fact the case. Following the ocular application of a 0.75% solution of porcine insulin, significant concentrations of insulin were demonstrable in the CSF extracted from the cisterna magnum, as well as in three brain regions. While it is not yet clear how insulin got into these target tissues, our data argue against a mechanism involving uptake from the blood (a fraction of topically applied compounds normally enters the vasculature through the conjunctiva and nasal mucosa). It is theorized that insulin may enter the CSF surrounding the optic nerve and by so doing, not only disseminate throughout the CSF space but also throughout the brain. The implications of these findings for central nervous system drug delivery are discussed.

Absorption↗

PubMed2003

Insulin and tropicamide accumulate in the contralateral, untreated eye of rats following ipsilateral topical administration by a mechanism that does not involve systemic uptake.

BACKGROUND: This study was designed to determine: (1) whether the accumulation of insulin in the contralateral retina and aqueous humor following ipsilateral topical insulin administration was due to systemic uptake and (2) whether tropicamide, applied to one eye, could induce dilation in the contralateral eye by a mechanism that did not involve systemic uptake. METHODS: Insulin eye drops were applied to the left eye of intact and decapitated rats, and their retinas and aqueous humors were then removed and their insulin levels quantified. In a separate experiment live animals received 0.1% tropicamide in their left eye and had their pupillary dilation response in both eyes measured at different time points. RESULTS: Administration of insulin to the left eye of decapitated rats resulted in its significant accumulation not only in the left retina and aqueous humor, but also in the retina and aqueous humor of the right eye. Similar aqueous humor results were obtained when live animals were used. Tropicamide drops induced marked pupillary dilation in treated eyes; the pupils of the contralateral, untreated eyes also dilated significantly, but less than did the treated pupils. The pupils of rats injected with tropicamide intravenously showed negligible dilation. CONCLUSIONS: These results showed that insulin accumulated in the retina and aqueous humor of contralateral, untreated eyes following topical application, by a mechanism that did not appear to involve systemic uptake. Similarly, tropicamide provoked a dilation response in the unheated eye by a mechanism that similarly did not appear to involve uptake from the blood.

Absorption↗