It is well known that vascular smooth muscle tone can be modulated by signals arising in the endothelium (e.g., endothelium-derived relaxing factor, endothelium-derived hyperpolarizing factor, and prostaglandins). Here we show that during vasoconstriction a signal can originate in smooth muscle cells and act on the endothelium to cause synthesis of endothelium-derived relaxing factor. We studied responses to two vasoconstrictors (phenylephrine and KCl) that act by initiating a rise in smooth muscle cell intracellular Ca2+ concentration ([Ca2+]i) and exert little or no direct effect on the endothelium. Fluo-3 was used as a Ca2+ indicator in either smooth muscle or endothelial cells of arterioles from the hamster cheek pouch. Phenylephrine and KCl caused the expected rise in smooth muscle cell [Ca2+]i that was accompanied by an elevation in endothelial cell [Ca2+]i. The rise in endothelial cell [Ca2+]i was followed by increased synthesis of NO, as evidenced by an enhancement of the vasoconstriction induced by both agents after blockade of NO synthesis. The molecule involved in signal transmission from smooth muscle to endothelium is as yet unknown. However, given that myoendothelial cell junctions are frequent in these vessels, we hypothesize that the rise in smooth muscle cell Ca2+ generates a diffusion gradient that drives Ca2+ through myoendothelial cell junctions and into the endothelial cells, thereby initiating the synthesis of NO.

Conducted vasodilation has been proposed as an important component of local vascular control. Because conducted vasomotor responses have previously been studied only in response to short pulses (<500 ms) of agonist, this study examined conducted vasodilation in response to sustained stimuli. In addition, we examined the contribution of nitric oxide (NO) to initiation and maintenance of conducted responses induced by acetylcholine (ACh). Responses to 2-min applications of ACh, sodium nitroprusside, and 8-bromoguanosine 3',5'-cyclic monophosphate were obtained in cannulated, perfused hamster cheek pouch arterioles (approximately 60 microm in diameter). Changes of luminal diameter in response to pressure ejection of agonists from a micropipette placed close to the downstream end of the vessel were observed at the site of stimulation ("local") as well as 570 and 1,140 microm upstream. At the local site, ACh stimuli produced large changes in diameter (approximately 70% of the maximum response) that peaked within 45 s before declining slowly to levels of approximately 50% of the maximum response. A similar response pattern was observed at both upstream sites, with the conducted responses being maintained for the duration of the stimulus. Local responses of similar magnitude were found with sodium nitroprusside and 8-bromoguanosine 3',5'-cyclic monophosphate, but only minimal responses were observed at the conducted sites. In a separate set of arterioles, ACh responses were obtained before and during perfusion with 10 microM N(omega)-nitro-L-arginine. Inhibition of NO synthesis diminished the local response to ACh, but the initial phase of the conducted response was unaffected. Furthermore, the conducted responses faded more rapidly in the presence of N(omega)-nitro-L-arginine. We conclude from these results that local NO synthesis alone is insufficient to initiate conducted responses but that NO synthesis contributes to maintenance of sustained conducted responses.

Many vasoactive agents, when applied to a discrete region of an arteriole, induce responses that spread bidirectionally along the vessel for distances that exceed those that can be accounted for by simple diffusion (6, 8, 17, 18). In the microcirculation, responses to agonists such as acetylcholine and phenylephrine conduct over distances of 2 mm or more (17). Based on the evidence summarized in Table 1, we have initiated a series of experiments designed to explore the hypothesis that conduction is an intrinsic property of the vessel wall and that it is likely an electrotonic spread of membrane potential change through gap junctions. The process of conduction thus represents a form of communication within the arteriolar wall that requires both inter- and intracellular signalling and that is thought to promote homogeneous flow regulation (5, 18). However, neither the intracellular signalling pathways that initiate the conducted signal nor the cell types that propagate the response have been elucidated.

Adenosine (Ado) is a potent vasodilator that has occasionally been shown to cause vasoconstriction. Constrictor responses are generally attributed to A1-receptor stimulation or interactions with the renin-angiotensin system. We describe a previously unreported vasoconstrictor action of Ado and inosine (Ino) in hamster cheek pouch arterioles and examine the mechanism by which these nucleosides induce constriction. Arterioles were dissected from male Golden hamster cheek pouches, transferred to a 37 degrees C tissue chamber, and cannulated at both ends. Changes of luminal diameter in response to Ado were measured to generate cumulative concentration-response curves. The concentration-response curves were biphasic: 10(-6) M Ado elicited an intense, transient constriction, and higher concentrations induced dilator responses. Pretreatment with 8(p-sulfophenyl)theophylline, an Ado receptor antagonist, inhibited the dilator responses but did not alter the constriction. Inhibition of Ado uptake with S-(4-nitrobenzyl)-6-thio-inosine eliminated the constrictor response without altering dilator responses. Similar effects were found after pretreatment with an Ado deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride. Finally, Ino, a metabolite of Ado, induced constrictions of similar magnitude to those seen with Ado, but at higher concentrations. The constrictor response was focal in nature, suggesting discrete sites of action of Ado. Methylene blue staining after Ado application revealed degranulated mast cells closely associated with the vessel wall, indicating a possible role for mast cell degranulation in the constrictor response. Supporting this idea were the observations that inhibition of degranulation by 10 microM cromolyn blocked the constrictor response, and compound 48/80 (a mast cell secretagogue) caused constriction similar to that elicited by Ado.(ABSTRACT TRUNCATED AT 250 WORDS)