Studying the influence of calcium-free medium and of reticular calcium levels upon contractility in isolated aorta, we previously established the depletion protocol. The stores sensitive to α-adrenergic stimulation partially overlap those mobilized by caffeine. We examined this aspect and the influence of extracellular magnesium in de-endothelialised rat aorta rings in isometric conditions (animals were killed by decapitation). Phenylephrine effects were observed in calcium-free medium (1 mM EDTA), when given after 30, 60 or 90 min incubation. We then examined the influence of caffeine (1 mM), ryanodine (0.01 mM) and various concentrations of magnesium (0 to 10 mM) upon the mentioned store-dependent contractions (results are % of the submaximal response in the same ring; mean ± S.E.M.; n = 6). The biphasic contraction in calcium-free medium is based upon two reticular compartments. The transient caffeine-sensitive response proved to be ryanodine sensitive; the store is exhausted after calcium-free incubation with EDTA for 60 min, as also shown by others. The second component, caffeine-insensitive, is abolished at 90 min incubation as above, though not affected in the first 60 min (P < 0.01; Student’s unpaired t test). Potentiation of the mentioned contractions by low magnesium was confirmed, while increased magnesium lowered the ratio between the transient and the sustained response. It has been suggested that reticular compartments communicate at rest. We suspect that depletion of the caffeine-sensitive one leads to redistribution, favouring depletion of the other. Functional tests in calcium-free medium are still useful, including the study of reticular compartments. Magnesium effects upon smooth muscle contractility are well characterized, yet this study provides particular evidence regarding the influence of extracellular magnesium upon the depletion of reticular calcium compartments in standardized conditions.

All procedures accord with current local guidelines.