The effect of a long-acting adenosine analog on blood flow through various organs in the dog

Effects of adenosine and its analogues on isolated intracerebral arterioles. Extraluminal and intraluminal application

We evaluated the responses of brain parenchymal arterioles to intraluminal and extraluminal application of adenosine and its analogues. Intracerebral arterioles (28.4- to 60.3-microns diameter) were isolated from Sprague-Dawley rats, cannulated with micropipettes, and perfused in vitro. Both extraluminal and intraluminal adenosine, 5'-(N-ethylcarboxamido)adenosine (NECA), R-N6-(phenylisopropyl)adenosine (R-PIA), and S-N6-(phenylisopropyl)adenosine (S-PIA) elicited concentration-dependent dilation of these arterioles, but intraluminal application was less potent and efficacious than extraluminal application. Inosine was not vasoactive. A common order of agonist potency (NECA > adenosine > R-PIA > or = S-PIA) was determined for both extraluminal and intraluminal application. Theophylline (10 microM) caused a rightward shift of the adenosine concentration-response curve and a 50-fold reduction in potency. Intraluminal theophylline was one sixth as effective as extraluminal theophylline in antagonizing the extraluminal adenosine response, whereas intraluminal 8-sulfophenyltheophylline, a polar theophylline derivative, was ineffective. Polyadenylic acid (PolyA, 1 microM), an adenosine polymer that does not penetrate the endothelium, induced a dilation of 44.2 +/- 5.3% when applied extraluminally but had no effect when infused intraluminally. The dilator effect of PolyA was antagonized by theophylline. We conclude that: (1) intraluminal adenosine and its analogues are effective dilators of intracerebral arterioles, (2) the dilator effects of both intraluminally and extraluminally applied adenosine are predominantly mediated by A2-type receptors, and (3) adenosine receptors mediating vasodilation are not present on the luminal surface of the endothelium.

Dynamic cerebral and systemic circulatory effects of adenosine, theophylline and dipyridamole

The effects of intravenous adenosine, dipyridamole and theophylline on local cerebral blood flow were studied in conscious rabbits. Long-term quantitative blood flow measurements were performed in 5 cerebral structures together with tissue pO2 and pCO2 measurements by a mass spectrometry technique. In an additional study, the time course of the cerebrovascular changes was determined by thermal clearance. It was found that: firstly, adenosine failed to modify local blood flow except in the caudate nucleus; secondly, dipyridamole increased cerebral blood flow in all 5 structures under study, and lastly, theophylline decreased cerebral blood flow in the same 5 structures. The increase in caudate blood flow induced by adenosine was instantaneous and lasted only for the duration of the infusion, whereas the cerebrovascular changes induced by dipyridamole and theophylline were gradual and persisted after their administration. Theophylline blocked the systemic and cerebrovascular changes caused by adenosine alone and by dipyridamole alone. In anesthetized rabbits, the intracarotid infusion of adenosine showed that the caudate reaction only occurred in the ipsilateral hemisphere. Taken together, our findings suggest that the transport system for adenosine in cerebral vessels is not only species-dependent but also structure-dependent. Furthermore, perivascular adenosine helps to maintain resting cerebrovascular tone and finally, cerebral adenosine may be involved in the control of cerebral blood flow via specific adenosine receptors.