Role of cyclic AMP and Ca2+ in potentiation of rat lacrimal gland protein secretion.

Addition of a cholinergic agonist carbachol and vasoactive intestinal peptide (VIP) to dispersed rat exorbital lacrimal gland acini produces protein secretion, measured by secretion of the enzyme peroxidase, that was statistically significantly greater than additive (potentiated). To determine where in stimulus-secretion coupling these secretagogues interact to potentiate secretion, rat exorbital gland acini were incubated simultaneously with cyclic AMP- and Ca2+-dependent agonists and protein secretion, cyclic AMP level, or Ca2+ concentration measured. As a measure of protein secretion, the supernatant obtained after centrifugation of acini was analyzed for peroxidase, a protein secreted by rat lacrimal glands. Interaction did not occur at the receptor level, because peroxidase secretion also was potentiated by simultaneous addition of carbachol and forskolin, which activates the catalytic subunit of adenyl cyclase. A potentiated increase in the cyclic AMP level did not potentiate protein secretion, because the level was the same with VIP as with carbachol and VIP added together at concentrations that potentiated peroxidase secretion. A potentiated increase in free intracellular [Ca2+] did not potentiate protein secretion, because [Ca2+] was greater with carbachol than with carbachol and VIP added together at concentrations that potentiated peroxidase secretion. We conclude that cholinergic- and VIP-dependent pathways interact to potentiate lacrimal gland protein secretion after the rise of intracellular cyclic AMP or Ca2+.

Interactions of NMDA antagonists and an alpha 2 agonist with mu, delta and kappa opioids in an acute nociception assay.

Animal and clinical studies have reported potentiation of opioid antinociception by co-administration of alpha-2 adrenoceptor agonists such as clonidine and NMDA receptor antagonists such as ketamine and dextromethorphan. The aim of this study was to compare these clinically available compounds in combination with classical morphine, fentanyl-like opioids, the delta opioid agonist SNC80 and the kappa opioid agonist U50,488H. Using a mouse hot-plate test, dose-response relationships were first determined for all compounds individually and then for opioids co-administered with fixed doses of clonidine, ketamine or dextromethorphan. Clonidine was also evaluated in combination with ketamine and dextromethorphan. ED50 values were calculated from the proportion of animals reaching a fixed cut-off criterion of 30 s. To varying degrees, all compounds produced increases in response latencies over time. Dextromethorphan produced lower ED50 values for morphine, fentanyl and sufentanil but exerted no effect on SNC80 or U50,488H. Similarly, ketamine potentiated the antinociceptive efficacy of morphine and sufentanil but not SNC80 or U50,488H. By contrast, clonidine potentiated all opioids tested. In addition, the potency of clonidine was found to increase with co-administration of ketamine but not dextromethorphan. The strongest opioid sparing interactions occurred between clonidine and the lipophilic mu opioids fentanyl and sufentanil and the delta opioid SNC80. In summary, these results suggest an important role for lipophilic opioids in combination therapies particularly with clonidine as well as possible advantages of specific delta or kappa opioid combinations with alpha-2 agonists.

Assessment of the plethysmographic variability index as a predictor of fluid responsiveness in critically ill patients: a pilot study.

Optimising intravascular volume in patients with hypotension requiring vasopressor support is a key challenge of critical care medicine. The optimal haemodynamic parameter to assess fluid responsiveness in critically ill patients, particularly those requiring a noradrenaline infusion and mechanical ventilation, remains uncertain. This pilot study assessed the accuracy of the plethysmographic variability index (PVI), (Radical-7 pulse co-oximeter, Masimo®, Irvine, CA, USA) in predicting fluid responsiveness in 25 patients who required noradrenaline infusion to maintain mean arterial pressure over 65 mmHg and were mechanically ventilated with a 'lung-protective' strategy, and whether administering a fluid bolus was associated with a change in PVI (Δ PVI). In this study, fluid responsiveness was defined as an increase in stroke volume of greater than 15% after a 500 ml bolus of colloid infusion over 20 minutes. Of the 25 patients included in the study, only 12 (48%) were considered fluid responders. As static haemodynamic parameters, PVI, central venous pressure and inferior vena cava distensibility index were all inaccurate at predicting volume responsiveness with PVI being the least accurate (area under the receiver operating characteristic curve=0.41, 95% confidence interval 0.18 to 0.65). However, fluid responsiveness was associated with a change in PVI, but not a change in heart rate or central venous pressure. This association between Δ PVI and fluid responsiveness may be a surrogate marker of improved cardiac output following a fluid bolus and warrants further investigation.

Vasoactive intestinal polypeptide stimulation of protein secretion from rat lacrimal gland acini.

The effect of vasoactive intestinal polypeptide (VIP) on protein secretion from lacrimal gland was investigated by using acini prepared by collagenase digestion of rat exorbital lacrimal glands. Protein secretion was determined by incubating the acini for 0-40 min and analyzing the supernatant for peroxidase, a protein secreted by the rat exorbital lacrimal gland. VIP (10(-10) to 10(-7) M) stimulated secretion in a concentration-dependent manner. A maximum concentration of VIP (10(-8) M) stimulated secretion to the same extent as a maximum concentration of carbachol (10(-5) M). The cholinergic antagonist atropine at a concentration (10(-5) M) that completely abolished carbachol-induced secretion did not alter VIP-stimulated secretion. The secretory effects of maximal concentrations of VIP and carbachol were additive, but decreasing the carbachol concentration potentiated secretion. Unlike carbachol, which had no effect on the acinar cAMP level, VIP increased cAMP content sixfold. Immunohistochemical staining demonstrated VIP-like immunoreactivity in nerve fibers throughout the gland, distributed primarily around acini. We conclude that VIP-like immunoreactive nerves are present in the lacrimal gland and that VIP can stimulate protein secretion but utilizes a pathway separate from, but convergent with, that used by cholinergic agonists.