In a previous study we have shown that magnesium (Mg²⁺) can modulate secretagogue-evoked secretory responses in the isolated parotid gland (Yago et al. 1999). This study investigated the effects of perturbation of extracellular magnesium ([Mg²⁺]_o) on secretagogue-induced secretory responses in the isolated rat submandibular gland. Donor rats were humanely killed by rapid cervical dislocation. The glands were isolated, cut into small segments (10-20 mg) and incubated in 20 ml of oxygenated Krebs-Henseleit solution for 30 min at 37 °C in a shaking water bath in the absence and presence of 10^-5 M acetylcholine (ACh), 10^-5 M noradrenaline (NA) or 10^-5 M phenylepinephrine (PHE) during perturbation (0 mM, 1.1 mM, 5 mM, 10 mM) of [Mg²⁺]_o. Following incubation, the tissues were removed, blotted dry and weighed. Total protein concentration in effluent samples was measured using a colorimetric method (Lowry et al. 1951) and expressed as µg ml^-1 (100 mg tissue)^-1; n = 6Ð8 except where stated. Submandibular acinar cells were isolated and loaded with fura-2 for the measurement of [Ca²⁺]_i using the cell suspension method (Lennard & Singh, 1992).

Basal protein outputs in zero, normal (1.1 mM) and elevated (5 mM and 10 mM) [Mg²⁺]_o were (means ± S.E.M.) 201.4 ± 11.7, 249.5 ± 13.3, 154.84 ± 11.3 and 175.1 ± 6.9, respectively. These results indicate that both zero and elevated [Mg²⁺]_o can reduce basal protein output compared with normal [Mg²⁺]_o. Stimulation of segments with 10^-5 M ACh, 10^-5 M NA or 10^-5 M PHE resulted in marked increases in total protein output in normal [Mg²⁺]_o compared with responses obtained in both zero and elevated [Mg²⁺]_o. Protein outputs in response to ACh, NA and PHE respectively were, in zero [Mg²⁺]_o, 203.7 ± 10.5, 320.9 ± 11.6 and 254.9 ± 8.9; in 1.1 mM [Mg²⁺]_o, 303.4 ± 22.7, 491.8 ± 21.7 and 319.9 ± 12.91; in 5 mM [Mg²⁺]_o, 205.1 ± 9.6, 216.7 ± 9 and 202.7 ± 10.1, and in in 10 mM [Mg²⁺]_o, 209.7 ± 30.7, 253.2 ± 16.3 and 213.2 ± 15.5. Basal [Ca²⁺]_i in zero, 1.1, 5.0 and 10.0 mM [Mg²⁺]_o was 106.2 ± 8.6, 104.5 ± 5.6, 47.7 ± 2.8 and 57.8 ± 8.7 nM (all n = 6Ð20), respectively. Stimulation of acinar cells with 10^-5 M ACh resulted in marked increases in both the initial peak and plateau phase of the Ca²⁺ transient in normal [Mg²⁺]_o. However, in zero and elevated [Mg²⁺]_o both the peak and the plateau phases of the ACh-induced Ca²⁺ transient were significantly (ANOVA plus post-hoc tests; P < 0.05) reduced compared with the responses obtained in normal [Mg²⁺]_o. The results indicate that Mg²⁺ can influence secretagogue-evoked secretory responses in the submandibular gland possibly by controlling Ca²⁺ mobilization.

All procedures accord with current UK legislation.