Resistance in blood vessels is directly related to the inner (luminal) diameter (ID). However, ID can be difficult to measure during physiological experiments because of poor transillumination of thick-walled or tightly constricted vessels. We investigated whether the wall cross-sectional area (WCSA) in cannulated arteries is nearly constant, allowing IDs to be calculated from outer diameters (OD) using a single determination of WCSA. With the use of image analysis, OD and ID were directly measured using either transillumination or a fluorescent marker in the lumen. IDs from a variety of vessel types were calculated from WCSA at several reference pressures. Calculated IDs at all of the reference WCSA were within 5% (mean <1%) of the corresponding measured IDs in all vessel types studied, including vessels from heterozygote elastin knockout animals. This was true over a wide range of transmural pressures, during treatment with agonists, and before and after treatment with KCN. In conclusion, WCSA remains virtually constant in cannulated vessels, allowing accurate determination of ID from OD measurement under a variety of experimental conditions.

A "caged" analogue of the a-adrenergic receptor agonist phenylephrine (PE) was prepared by exploiting the 2-nitrobenzyl protecting group and using a synthetic procedure developed to permit preferential derivatization at the amino group. On isolated adult rat mesenteric arterioles, caged-PE had no measurable effects at concentrations up to 100 IAM; 0.5-ms light flashes in the presence of caged-PE, however, produced marked and dose-dependent vasoconstriction. Flashinduced vasoconstrictions were blocked by the a-receptor antagonist phentolamine and were unaffected by the 13-receptor antagonist propranolol, indicating that the light-induced responses reflect the selective activation of a-adrenergic receptors. After a single flash, a large transient decrease in vessel diameter was recorded, and in most vessels, this was followed by a smaDler, sustained constriction. The sustained component of the contraction was selectively eliminated when Ca2+ was removed from the bath, which suggests that different mechanisms underlie the transient and the sustained responses to PE. The responses to single flashes of varying intensities occurred with a mean latency of 460 ms, which is consistent with the intermediacy of several steps between a-receptor activation and contraction. We anticipate that it will be possible to extend this approach to develop caged analogues of other neurotransmitters for mechanistic and kinetic studies.Previously, this laboratory and others (1-3) have exploited the 2-nitrobenzyl protecting group (see Fig. 1) in the development of "caged" analogues of a variety of intracellular second messengers. The rationale for this approach was the anticipated spatial and temporal resolution afforded by controlling the intracellular messenger application with light, as compared to more conventional techniques. There are, however, a number of criteria that a caged compound need fulfill to be useful in biological experiments (1-3). First, the caged analogue should lack biological activity, so that it can be applied under steady-state conditions. Second, photorelease should proceed with reasonable quantum efficiency and be devoid of competing or side reactions. Third, photorelease should be fast and irreversible, preferably being complete in '1 ms. Finally, any side products produced on photolysis should be chemically inert, biologically inactive, and nontoxic. Given the success of this approach in providing caged intracellular second messengers (1-3) and our interest in the mechanisms involved in the modulation of voltage-gated ion channels by adrenergic receptor stimulation (4-6), we reasoned that this methodology could be exploited to develop caged adrenergic receptor agonists. In support of the general applicability of this approach, the 2-nitrobenzyl moiety has recently been used in the development of caged analogues of carbamylcholine, a nicotinic cholinergic receptor agonist (7,8).The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marke...

Halothane has endothelium-independent vasoconstricting and vasodilating actions in isolated mesenteric resistance blood vessels. The vasoconstricting action appears to involve halothane-induced Ca2+ release from caffeine/ryanodine-sensitive intracellular store(s). The vasodilating action in phenylephrine- or KC1-constricted vessels is independent of the Ca(2+)-releasing action and most likely involves an effect(s) on sarcolemmal-dependent Ca2+ signaling (e.g., extracellular Ca2+ influx) and/or Ca2+ activation of contractile proteins. The magnitude of both the vasoconstricting and the vasodilating actions of halothane in these vessels at clinically relevant concentrations suggests these direct actions contribute to the overall cardiovascular effects of halothane in vivo.

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