Effect of adenosine on calcium uptake by intact and cultured vascular smooth muscle

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Abstract
The effect of adenosine (ADO), a potent vasodilator, on the cellular Ca uptake by vascular smooth muscle (VSM) was studied. Similar experiments were conducted with other vasoactive agents to ensure the validity of results obtained with the cultured VSM cell model. Primary VSM monolayers from rat aorta were incubated for 30 min in 45Ca-enriched physiological salt solution (PSS) (37.degree. C) and washed with 20 mM Ca-EGTA [ethyleneglycol-bis(.beta.-aminoethylether)N,N,N'',N''-tetraacetic acid] (4.degree. C) to remove extracellular 45Ca. Ca uptake with 4.6 mM K+ was 65.3 .+-. 2.4 pmol Ca/105 cells. Verapamil (10-6 M) or 4.degree. C incubation decreased with value 27 and 65%, respectively, whereas ADO (10-7 M) and isoproterenol (10-6 M) elevated Ca uptake 21 and 9%, respectively. Elevation of extracellular K+ (25 mM) increased Ca uptake to 87.8 .+-. 5.0 pmol Ca/105 cells. ADO (10-7 M), isoproterenol (10-6 M) and 4.degree. C incubation significantly attenuated this K+-induced increase 25, 19 and 64%, respectively. The Ca uptake of nondepolarized cells was not changed by norepinephrine (10-6 M) in the presence of either .alpha.- and .beta.-adrenergic blockade. Experiments were performed in a similar manner with porcine carotid artery strips in the presence of .alpha.- and .beta.-adrenergic blockade. Ca uptake increased from 2.3 .+-. 0.2 .times. 10-7 to 3.0 .+-. 0.1 .times. 10-7 mol Ca/g wt/wt after elevation of the medium K+ to 25 mM. This increase was inhibited by 10-6 M ADO. ADO (10-6 M) had no effect on the 45Ca efflux from cultured VSM. Apparently, ADO relaxes VSM by reducing the inward movement of Ca during stimulation. The significant ADO-induced elevation of cellular Ca under conditions when cultured cells were not partially depolarized suggests the existence of an enhanced Ca sequestration within these cells.

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