Regulation of free arachidonic acid levels in isolated salivary glands from the lone star tick: A role for dopamine

Abstract

An important regulatory step for prostaglandin synthesis is the availability of the precursor, free arachidonic acid (AA). In isolated salivary glands of the lone star tick, Amblyomma americanum (L.), the level of free AA appears to depend on higher phospholipase A2 (PLA2) activity rather than decreased rates of re-esterification by lysophosphatide acyl transferase (LAT). This conclusion is supported by experiments where inhibition of LAT with merthiolate was without effect, while the calcium ionophore A23187, a PLA2 stimulant, increased levels of free AA. The PLA2 activity in A. americanum was reduced by the substrate analog, PLA2 inhibitor, oleyloxyethyl phosphorylcholine in a dose-dependent manner, but was insensitive to the other mammalian PLA2 inhibitors mepacrine (20 microM), aristolochic acid (45 microM), and dexamethasone (50 microM). No substrate preference was observed for the functional group of the phospholipid, with phosphatidylcholine and phosphatidylethanolamine being equal sources of AA in A23187-stimulated glands. Compared to phospholipids containing other fatty acids, only arachidonyl-phospholipid (arachidonyl-PL) was significantly hydrolyzed by PLA2 activity in A23187-stimulated glands. Dopamine was as effective as A23187 as a stimulant of PLA2 activity in isolated glands, but this effect was abolished in the presence of the calcium channel blocking agent verapamil. It is concluded that free AA levels in tick salivary glands are increased through activation of a Type IV-like PLA2 following an increase of intracellular calcium caused by the opening of voltage-dependent calcium channels due to dopamine stimulation.

A specific prostaglandin E2 receptor and its role in modulating salivary secretion in the female tick, Amblyomma americanum (L.)

“…Arachidonic acid is released from phopholipids after stimulation of tick salivary glands by dopamine (Bowman et al, 1995a). Treatment of isolated salivary glands with inhibitors of eicosanoid biosynthesis OPC, ETY A, and DEX, significantly reduced dopamine-induced fluid secretion, suggesting a role for eicosanoids in regulating secretion.…”

“…In addition, the cyclo-oxygenase inhibitor indomethacin at 1 mM inhibited cAMP accumulation about 27%. Verapamil, which inhibits dopamine-stimulated secretion by blocking Ca 2 + influx and also blocks arachidonic acid release (Bowman et al, 1995a), also reduced the cAMP accumulation about 65% at 1 mM (Table 2.2). In the presence of OPC, PGE2 and its analog 17-phenyl trinor PGE2 stimulated cAMP accumulation about 20% and 40% above the OPC inhibited control, respectively ( Fig.…”

“…The best documented agonist ofsecretion is dopamine, a neurotransmitter released at the neuroeffector junction, to initiate salivary secretion (Sauer et al, 1995). Since dopamine can also stimulate the release of arachidonic acid from salivary gland phospholipids (Bowman et al, 1995a) which may then be metabolized into prostaglandins, can prostaglandins ( e.g. PGE2) function in regulation oftick salivary secretion in the same manner as in other tissues?…”

“…It is interesting to note that dopamine also stimulates the release of arachidonic acid from phospholipids in the tick salivary glands by presumably activating an intracellular PLA2 (Bowman et al, 1995a). Free arachidonic acid may be converted into PGs via the cyclooxygenase pathway in tick salivary glands (Higgs et al, 1976;Dickenson et al, 1976;Shemesh et al, 1979;Ribeiro et al, 1985Ribeiro et al, , 1988Ribeiro et al, , 1992Bowman et al, 1993;Shipley et al, 1993).…”

“…PGE2) are typically synthesized from arachidonic acid (AA) after AA is released from cellular phospholipids following activation of an intracellular phospholipase A2 (PLA2). Dopamine stimulates release of AA from salivary gland phospholipids presumably by activating an intracellular PLA2 (Bowman et al, 1995a). Treatment of isolated salivary glands with PLA2 inhibitor oleyloxyethyl phosphorylcholine (OPC) or prostaglandin synthetase inhibitors reduces dopamineinduced fluid secretion and cAMP levels in isolated salivary glands.…”