Online Detection of Catecholamine Release from the Perfused Rat Adrenal Gland

Retrogradely perfused adrenal glands have historically served for establishing many of our current knowledge on the stimulus-secretion coupling process. Although the use of intact adrenals has largely been switched to isolated chromaffin cells, adrenal glands are still a very valuable tool to characterize physiological and pharmacological questions. Even more, this is an excellent preparation for studying the splanchnic nerve/chromaffin cell interaction. In this chapter, we will provide the ways to (i) perform retrograde perfusion of isolated rat adrenals, (ii) the method to apply electrical splanchnic nerve stimulation, and (iii) the preparation of adrenals to conduct online electrochemical detection of catecholamine release.

Real Time Recording of Perifused Chromaffin Cells

Amperometry is an electrochemical method based on the oxidation or reduction of molecules. Many secretion products, including catecholamines, contain in their molecule chemical groups with the ability to yield (oxidize) or capture (reduce) electrons upon its exposure to an electrical field. In order to measure the secretion of catecholamines, they are oxidized at +650 mV with a carbon electrode, releasing every molecule of catecholamine that is oxidized two electrons (e^-) that are recorded as an electrical current. Amperometry is an easy-to-use and noninvasive technique for cells (unlike patch-clamp techniques for measuring membrane capacitance) and has been widely used to monitor online catecholamine release from perifused bovine chromaffin cell populations.

5 ns electric pulses induce Ca2+-dependent exocytotic release of catecholamine from adrenal chromaffin cells

Previously we reported that adrenal chromaffin cells exposed to a 5 ns, 5 MV/m pulse release the catecholamines norepinephrine (NE) and epinephrine (EPI) in a Ca²⁺-dependent manner. Here we determined that NE and EPI release increased with pulse number (one versus five and ten pulses at 1 Hz), established that release occurs by exocytosis, and characterized the exocytotic response in real-time. Evidence of an exocytotic mechanism was the appearance of dopamine-β-hydroxylase on the plasma membrane, and the demonstration by total internal reflection fluorescence microscopy studies that a train of five or ten pulses at 1 Hz triggered the release of the fluorescent dye acridine orange from secretory granules. Release events were Ca²⁺-dependent, longer-lived relative to those evoked by nicotinic receptor stimulation, and occurred with a delay of several seconds despite an immediate rise in Ca²⁺. In complementary studies, cells labeled with the plasma membrane fluorescent dye FM 1-43 and exposed to a train of ten pulses at 1 Hz underwent Ca²⁺-dependent increases in FM 1-43 fluorescence indicative of granule fusion with the plasma membrane due to exocytosis. These results demonstrate the effectiveness of ultrashort electric pulses for stimulating catecholamine release, signifying their promise as a novel electrostimulation modality for neurosecretion.

Roles of Na+, Ca2+, and K+ channels in the generation of repetitive firing and rhythmic bursting in adrenal chromaffin cells

Adrenal chromaffin cells (CCs) are the main source of circulating catecholamines (CAs) that regulate the body response to stress. Release of CAs is controlled neurogenically by the activity of preganglionic sympathetic neurons through trains of action potentials (APs). APs in CCs are generated by robust depolarization following the activation of nicotinic and muscarinic receptors that are highly expressed in CCs. Bovine, rat, mouse, and human CCs also express a composite array of Na⁺, K⁺, and Ca²⁺ channels that regulate the resting potential, shape the APs, and set the frequency of AP trains. AP trains of increasing frequency induce enhanced release of CAs. If the primary role of CCs is simply to relay preganglionic nerve commands to CA secretion, why should they express such a diverse set of ion channels? An answer to this comes from recent observations that, like in neurons, CCs undergo complex firing patterns of APs suggesting the existence of an intrinsic CC excitability (non-neurogenically controlled). Recent work has shown that CCs undergo occasional or persistent burst firing elicited by altered physiological conditions or deletion of pore-regulating auxiliary subunits. In this review, we aim to give a rationale to the role of the many ion channel types regulating CC excitability. We will first describe their functional properties and then analyze how they contribute to pacemaking, AP shape, and burst waveforms. We will also furnish clear indications on missing ion conductances that may be involved in pacemaking and highlight the contribution of the crucial channels involved in burst firing.

Regulation by L channels of Ca(2+)-evoked secretory responses in ouabain-treated chromaffin cells

It is known that the sustained depolarisation of adrenal medullary bovine chromaffin cells (BCCs) with high K(+) concentrations produces an initial sharp catecholamine release that subsequently fades off in spite depolarisation persists. Here, we have recreated a sustained depolarisation condition of BCCs by treating them with the Na(+)/K(+) ATPase blocker ouabain; in doing so, we searched experimental conditions that permitted the development of a sustained long-term catecholamine release response that could be relevant during prolonged stress. BCCs were perifused with nominal 0Ca(2+) solution, and secretion responses were elicited by intermittent application of short 2Ca(2+) pulses (Krebs-HEPES containing 2 mM Ca(2+)). These pulses elicited a biphasic secretory pattern with an initial 30-min period with secretory responses of increasing amplitude and a second 30-min period with steady-state, non-inactivating responses. The initial phase was not due to gradual depolarisation neither to gradual increases of the cytosolic calcium transients ([Ca(2+)]c) elicited by 2Ca(2+) pulses in BBCs exposed to ouabain; both parameters increased soon after ouabain addition. Νifedipine blocked these responses, and FPL64176 potentiated them, suggesting that they were triggered by Ca(2+) entry through non-inactivating L-type calcium channels. This was corroborated by nifedipine-evoked blockade of the L-type Ca(2+) channel current and the [Ca(2+)]c transients elicited by 2Ca(2+) pulses. Furthermore, the plasmalemmal Na(+)/Ca(2+) exchanger (NCX) blocker SEA0400 caused a mild inhibition followed by a large rebound increase of the steady-state secretory responses. We conclude that these two phases of secretion are mostly contributed by Ca(2+) entry through L calcium channels, with a minor contribution of Ca(2+) entry through the reverse mode of the NCX.

Comparative analysis of efficacy of lignocaine 1.5 mg/kg and two different doses of dexmedetomidine (0.5 μg/kg and 1 μg/kg) in attenuating the hemodynamic pressure response to laryngoscopy and intubation

CONTEXT

Laryngoscopy and intubation cause an intense reflex increase in heart rate, blood pressure, due to an increased sympathoadrenal pressor response. Lignoocaine has shown blunting of pressor response to intubation. Dexmedetomidine has sympatholytic effects.

AIMS

To the best of our knowledge there is no study comparing the efficacy of lignocaine with two different doses of dexmedetomidine for attenuating the pressor response. With this idea, we planned to conduct the present study.

MATERIALS AND METHODS

After approval by the Hospital Ethics committee, 90 consenting adults aged 18-65 years of age of either sex of non-hypertensive ASA Grade I or II were randomly allocated into three groups. Group D1- IV Dexmedetomidine 0.5μg/kg over 10 minutes Group D2- IV Dexmedetomidine 1μg/kg over 10 minutes Group X- IV Lignocaine 1.5mg/kg in 10 ml normal saline.

STATISTICAL ANALYSIS USED

ANOVA and Student's t test used for analysis.

RESULTS

Dexmedetomidine 1µg/kg was more effective than 0.5µg/kg and lignocaine 1.5mg/kg in attenuating the pressor response.

CONCLUSIONS

We conclude that dexmedetomidine 1μg/kg adequately attenuates the hemodynamic response to laryngoscopy and endotracheal intubation when compared with dexmedetomidine 0.5μg/kg and lignocaine 1.5mg/kg.