Tubocurarine blocks a calcium-dependent potassium current in rat tumoral pituitary cells

Large conductance voltage-and calcium-activated K+ (BK) channel in health and disease

Large Conductance Voltage- and Calcium-activated K+ (BK) channels are transmembrane pore-forming proteins that regulate cell excitability and are also expressed in non-excitable cells. They play a role in regulating vascular tone, neuronal excitability, neurotransmitter release, and muscle contraction. Dysfunction of the BK channel can lead to arterial hypertension, hearing disorders, epilepsy, and ataxia. Here, we provide an overview of BK channel functioning and the implications of its abnormal functioning in various diseases. Understanding the function of BK channels is crucial for comprehending the mechanisms involved in regulating vital physiological processes, both in normal and pathological conditions, controlled by BK. This understanding may lead to the development of therapeutic interventions to address BK channelopathies.

d-Tubocurarine and Berbamine: Alkaloids That Are Permeant Blockers of the Hair Cell's Mechano-Electrical Transducer Channel and Protect from Aminoglycoside Toxicity

Aminoglycoside antibiotics are widely used for the treatment of life-threatening bacterial infections, but cause permanent hearing loss in a substantial proportion of treated patients. The sensory hair cells of the inner ear are damaged following entry of these antibiotics via the mechano-electrical transducer (MET) channels located at the tips of the hair cell's stereocilia. d-Tubocurarine (dTC) is a MET channel blocker that reduces the loading of gentamicin-Texas Red (GTTR) into rat cochlear hair cells and protects them from gentamicin treatment. Berbamine is a structurally related alkaloid that reduces GTTR labeling of zebrafish lateral-line hair cells and protects them from aminoglycoside-induced cell death. Both compounds are thought to reduce aminoglycoside entry into hair cells through the MET channels. Here we show that dTC (≥6.25 μM) or berbamine (≥1.55 μM) protect zebrafish hair cells in vivo from neomycin (6.25 μM, 1 h). Protection of zebrafish hair cells against gentamicin (10 μM, 6 h) was provided by ≥25 μM dTC or ≥12.5 μM berbamine. Hair cells in mouse cochlear cultures are protected from longer-term exposure to gentamicin (5 μM, 48 h) by 20 μM berbamine or 25 μM dTC. Berbamine is, however, highly toxic to mouse cochlear hair cells at higher concentrations (≥30 μM) whilst dTC is not. The absence of toxicity in the zebrafish assays prompts caution in extrapolating results from zebrafish neuromasts to mammalian cochlear hair cells. MET current recordings from mouse outer hair cells (OHCs) show that both compounds are permeant open-channel blockers, rapidly and reversibly blocking the MET channel with half-blocking concentrations of 2.2 μM (dTC) and 2.8 μM (berbamine) in the presence of 1.3 mM Ca²⁺ at −104 mV. Berbamine, but not dTC, also blocks the hair cell's basolateral K⁺ current, I_K,neo, and modeling studies indicate that berbamine permeates the MET channel more readily than dTC. These studies reveal key properties of MET-channel blockers required for the future design of successful otoprotectants.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Voltage-dependent ionic conductances in the human malignant astrocytoma cell line U87-MG

Although the human malignant astrocytoma cell line U87-MG has been used in numerous studies, few findings are available on the properties of its membrane ion conductances. Characterization of the ion channels expressed in these cells will make it possible to study membrane ion conductance changes when a receptor is activated by its ligand. This will help to elucidate the functional properties of these receptors and their signal-transduction pathways in pathophysiological events. This work studied the voltage-dependent ionic conductances of U87-MG cells using the Whole-Cell Recording patch-clamp technique. Six types of voltage-dependent ionic currents were identified: (i) a TEA-, 4-AP- and CTX-sensitive Ca2+-dependent K+ current, (ii) a transient K+ current inhibited by 4-AP, (iii) an inwardly rectifying K+ current blocked by Ba2+ and 4-AP, (iv) a DIDS- and SITS-sensitive Cl- current, (v) a TTX-sensitive Na+ conductance and (vi) a L-type Ca2+ conductance activated by BayK-8644 and inhibited by Ni and the L-type Ca2+ channel inhibitor, nifedipine. In addition, electrical depolarizations elicited inward currents due to voltage-independent, Ca2+-dependent K+ influx against the electrochemical gradient, probably via an ouabain-sensitive Na+-K+ pump.

Ca2+-activated K+ channels in gonadotropin-releasing hormone-stimulated mouse gonadotrophs

GnRH receptor activation elicits release of intracellular Ca(2+), which leads to secretion and also activates Ca(2+)-activated ion channels underlying membrane voltage changes. The predominant Ca(2+)-activated ion channels in rat and mouse gonadotrophs are Ca(2+)-activated K(+) channels. To establish the temporal relationship between GnRH-induced changes in intracellular [Ca(2+)] ([Ca(2+)](i)) and membrane current (I(m)), and to identify specific Ca(2+)-activated K(+) channels linking GnRH-induced increase in [Ca(2+)](i) to changes in plasma membrane electrical activity, we used single female mouse gonadotrophs in the perforated patch configuration of the patch-clamp technique, which preserves signaling pathways. Simultaneous measurement of [Ca(2+)](i) and I(m) in voltage-clamped gonadotrophs revealed that GnRH stimulates an increase in [Ca(2+)](i) that precedes outward I(m), and that activates two kinetically distinct currents identified, using specific toxin inhibitors, as small conductance Ca(2+)-activated K(+) (SK) current (I(SK)) and large (big) conductance voltage- and Ca(2+)-activated K(+) (BK) current (I(BK)). We show that the apamin-sensitive current has an IC(50) of 69 pM, consistent with the SK2 channel subtype and confirmed by immunocytochemistry. The magnitude of the SK current response to GnRH was attenuated by 17beta-estradiol (E(2)) pretreatment. Iberiotoxin, an inhibitor of BK channels, completely blocked the residual apamin-insensitive outward I(m), substantiating that I(BK) is a component of the GnRH-induced outward I(m). In contrast to its suppression of I(SK), E(2) pretreatment augmented peak I(BK). SK or BK channel inhibition modulated GnRH-stimulated LH secretion, implicating a role for these channels in gonadotroph function. In summary, in mouse gonadotrophs the GnRH-stimulated increase in [Ca(2+)](i) activates I(SK) and I(BK), which are differentially regulated by E(2) and which may be targets for E(2) positive feedback in LH secretion.

A functional role of the erg-like inward-rectifying K+ current in prolactin secretion from rat lactotrophs

The functional role of the inward-rectifying erg-like K+ current in rat lactotrophs was studied by the use of a selective blocker, the class III antiarrhythmic agent E-4031. The erg-like current was measured as drug-sensitive current in physiological K+ gradient. In the range of the normal resting membrane potential of rat lactotrophs (around -45 mV) the erg-like current constituted a steady outward current. A selective block of this current by E-4031 resulted in a moderate (5 mV) depolarization of the membrane potential in 64% of the lactotroph cells. Measurements of basal prolactin secretion with the reverse hemolytic plaque assay showed that the number of prolactin secreting cells and the amount of prolactin secreted from single lactotrophs was significantly increased in the presence of E-4031. The data show that the contribution of the erg-like K+ current to the maintenance of the resting membrane potential is functionally important for the regulation of prolactin secretion.