Natriuretic peptides and the control of catecholamine release in two freshwater teleost and a marine elasmobranch fish

Experiments were carried out in situ and in vivo to investigate the relationship between natriuretic peptides (NPs) and humoral catecholamine secretion in the American eel (Anguilla rostrata), rainbow trout (Oncorhynchus mykiss) and spiny dogfish (Squalus acanthias). In situ perfusion of the chromaffin tissue of A. rostrata with homologous atrial NP (ANP; 10–9 mol l–1) or ventricular NP (VNP; 10–9 mol l–1), or O. mykiss with either rat ANP (10–9 mol l–1), eel VNP (10–9 mol l–1), or trout VNP (10–9 mol l–1), did not significantly affect basal or carbachol-elicited (10–5 mol kg–1) catecholamine release. Bolus injections of homologous ANP (10–9 mol kg–1) or VNP (10–9 mol kg–1) in A. rostrata in vivo elicited a rapid and prolonged reduction in arterial blood pressure and an increase in heart rate (fH); circulating plasma catecholamine levels were unaffected. In O. mykiss, bolus injections of rat ANP (10–9 mol kg–1) or trout VNP (10–9 mol kg–1) elicited a significant bi-phasic pressor- depressor response and a marked increase in fH. Neither the acute pressor or the longer-term depressor effects of NPs in O. mykiss were associated with any change in circulating plasma catecholamine levels. In S. acanthias, bolus injections of homologous C-type natriuretic peptide (CNP; 10–9 mol kg–1) elicited a bi-phasic pressor-depressor response, an increase in systemic resistance, a decrease in cardiac output and stroke volume, but no change in fH. Plasma noradrenaline levels were elevated 15- fold after CNP injection while circulating adrenaline levels remained unchanged. These results show that NPs of systemic origin do not directly or indirectly affect basal or cholinergically-mediated catecholamine release in A. rostrata and O. mykiss and that the initial pressor response to NP's in trout cannot be attributed to an elevation of circulating catecholamines. Conversely, CNP appears to be a potent secretagogue (direct or indirect) of noradrenaline release in S. acanthias and thus there is likely to be a significant humoral adrenergic component to the cardiovascular effects of NPs in this species.     

Effect of three treatment protocols on acute ocular hypertension after phacoemulsification and aspiration of cataracts in dogs

Objective  To compare the effect of topical latanoprost, intracameral carbachol, or no adjunctive medical therapy on the development of acute postoperative hypertension (POH) and inflammation after routine phacoemulsification and aspiration (PA) of cataracts in dogs.
Design  Retrospective study.
Procedures  Dogs received either one drop of topical 0.005% latanoprost (21 dogs, 39 eyes), an intracameral injection of 0.3 mL of 0.01% carbachol (15 dogs, 30 eyes), or no adjunctive therapy (46 dogs, 90 eyes) immediately following PA of cataract(s). Intraocular pressure (IOP) was measured in all dogs 2 and 4 h after surgery. IOP was measured and aqueous flare assessed at 8 am the day after surgery.
Results  Carbachol-treated dogs had significantly higher mean IOP (33.2 ± SD 20.8 mmHg) 2 h after surgery than dogs receiving no adjunctive therapy (22.0 ± SD 14.1 mmHg) ( P  =  0 .049). There were no significant differences in IOP among groups at any other time point. There were no significant differences in number of POH episodes between dogs treated with carbachol (47%), latanoprost (29%), or dogs that received no adjunctive therapy (33%). There were no significant differences in mean aqueous flare grade between eyes treated with latanoprost (1.7 ± SD 0.4) or carbachol (1.4 ± SD 0.6), and eyes that received no adjunctive therapy (1.7 ± SD 0.4).
Conclusions  Topical 0.005% latanoprost or intracameral injection of 0.3 mL of 0.01% carbachol after PA in dogs did not reduce POH or increase intraocular inflammation compared to dogs not receiving adjunctive therapy after PA of cataracts.     

Effects of calcium channel blockers on pharmacologically induced contractions of rainbow trout (Oncorhynchus mykiss) intestine

Calcium depletion/replacement studies were carried out to examine the role of calcium in contraction of trout intestinal smoot muscle in vitro. Three chemically distinct calcium channel blockers were used to determine whether voltage operated calcium channels (VOCs) were involved in calcium entry with either agonist or depolarization-induced contractions. Contractions induced by depolarizing intestinal smooth muscle with potassium were totally dependent on extracellular calcium, whereas receptor-mediated responses to 5-hydroxytryptamine (5-HT) and carbachol also relied on calcium derived from intracellular stores. The calcium channel blockers, verapamil, nitrendipine, and diltiazem, all shifted the calcium-response curve for potassium to the right, supporting the existence of VOCs in trout intestinal smooth muscle. The calcium-response curve for 5-HT was also shifted to the right, suggesting that 5-HT can induce calcium uptake into the smooth muscle via VOCs, in addition to mobilizing intracellular calcium. Verapamil also appeared to block 5-HT receptors directly. Carbachol-induced contractions were only reduced by diltiazem at low concentrations of calcium (0.1–1 mM), suggesting that diltiazem has some other mechanisms of action than the other calcium channel blockers. Activation of muscarinic receptors may induce calcium entry through channels other than the VOCs, in addition to mobilizing intracellular calcium.     

Muscarinic M3-like receptors, cyclic AMP and L-type calcium channels are involved in the contractile response to cholinergic agents in gut smooth muscle of the rainbow trout, Oncorhynchus mykiss

Cholinergic signalling in mammalian gut smooth muscle usually involves M3 muscarinic receptors for direct contraction via phospholipase C activation and M2 muscarinic receptors to reduce cyclic AMP levels. However, the proportion of receptor subtypes and second messengers involved varies among tissues and animals and studies in non-mammalian species will provide information on the conservation of pathways and consequently on their importance for signal transduction. In the present study we investigated receptor subtypes, involvement of calcium, phospholipase C and cyclic AMP in the cholinergic contraction of the rainbow trout gut. Intestinal and gastric smooth muscle strip preparations, with the mucosa removed, were used in functional studies, and homogenised strips were used for measurements of cyclic AMP. Calcium-free medium, the L-type calcium-channel inhibitor verapamil, the cyclic AMP-phosphodiesterase inhibitor IBMX + isoprenaline, and the M3-antagonist 4-DAMP methiodide all caused a partial or marked reduction of the response to cholinergic agonists. Neomycin, an inhibitor of phospholipase C, and SKF96365, an inhibitor of receptor-operated calcium channels, had no effect. Carbachol (0.1 mM) reduced the levels of cyclic AMP transiently. It is concluded that the cholinergic signal transduction in rainbow trout gut smooth muscle involves (1) binding to M3-like receptors, (2) a transient reduction in cyclic AMP levels, (3) influx of extracellular calcium, in part through L-type calcium-channels and (4) no involvement of phospholipase C.