Different sodium requirements for 86Rb efflux and for growth hormone and prolactin secretion from bovine anterior pituitary cells

Sodium background currents in endocrine/neuroendocrine cells: Towards unraveling channel identity and contribution in hormone secretion

In endocrine/neuroendocrine tissues, excitability of secretory cells is patterned by the repertoire of ion channels and there is clear evidence that extracellular sodium (Na⁺) ions contribute to hormone secretion. While voltage-gated channels involved in action potential generation are well-described, the background 'leak' channels operating near the resting membrane potential are much less known, and in particular the channels supporting a background entry of Na⁺ ions. These background Na⁺ currents (called here 'I_Nab') have the ability to modulate the resting membrane potential and subsequently affect action potential firing. Here we compile and analyze the data collected from three endocrine/neuroendocrine tissues: the anterior pituitary gland, the adrenal medulla and the endocrine pancreas. We also model how I_Nab can be functionally involved in cellular excitability. Finally, towards deciphering the physiological role of I_Nab in endocrine/neuroendocrine cells, its implication in hormone release is also discussed.

A sodium background conductance controls the spiking pattern of mouse adrenal chromaffin cells in situ

KEY POINTS

Mouse chromaffin cells in acute adrenal slices exhibit two distinct spiking patterns, a repetitive mode and a bursting mode. A sodium background conductance operates at rest as demonstrated by the membrane hyperpolarization evoked by a low Na⁺ -containing extracellular saline. This sodium background current is insensitive to TTX, is not blocked by Cs⁺ ions and displays a linear I-V relationship at potentials close to chromaffin cell resting potential. Its properties are reminiscent of those of the sodium leak channel NALCN. In the adrenal gland, Nalcn mRNA is selectively expressed in chromaffin cells. The study fosters our understanding of how the spiking pattern of chromaffin cells is regulated and adds a sodium background conductance to the list of players involved in the stimulus-secretion coupling of the adrenomedullary tissue.

ABSTRACT

Chromaffin cells (CCs) are the master neuroendocrine units for the secretory function of the adrenal medulla and a finely-tuned regulation of their electrical activity is required for appropriate catecholamine secretion in response to the organismal demand. Here, we aim at deciphering how the spiking pattern of mouse CCs is regulated by the ion conductances operating near the resting membrane potential (RMP). At RMP, mouse CCs display a composite firing pattern, alternating between active periods composed of action potentials spiking with a regular or a bursting mode, and silent periods. RMP is sensitive to changes in extracellular sodium concentration, and a low Na⁺ -containing saline hyperpolarizes the membrane, regardless of the discharge pattern. This RMP drive reflects the contribution of a depolarizing conductance, which is (i) not blocked by tetrodotoxin or caesium, (ii) displays a linear I-V relationship between -110 and -40 mV, and (iii) is carried by cations with a conductance sequence g_Na  > g_K  > g_Cs . These biophysical attributes, together with the expression of the sodium-leak channel Nalcn transcript in CCs, state credible the contribution of NALCN. This inaugural report opens new research routes in the field of CC stimulus-secretion coupling, and extends the inventory of tissues in which NALCN is expressed to neuroendocrine glands.

A role for a background sodium current in spontaneous action potentials and secretion from rat lactotrophs

We have used the perforated-patch variation of whole cell patch-clamp techniques, measurements of cytosolic calcium with use of fura 2, and secretion measurements with use of the reverse-hemolytic plaque assay to address the role of depolarizing background currents in maintaining spontaneous action potentials and spontaneous secretion from rat lactotrophs in primary culture. Replacement of bath sodium with tris(hydroxymethyl)aminomethane or N-methyl-D-glucamine caused a dramatic hyperpolarization of the cells, a cessation of spontaneous action potentials, and an increase in input resistance of cells. Tetrodotoxin had no effect on spontaneous action potentials, and removal of bath calcium stopped spiking but did not hyperpolarize the cells. The hyperpolarization in response to removal of bath sodium was associated with a decrease in cytosolic calcium levels. Finally, removal of bath sodium caused a decrease in spontaneous secretion of prolactin from lactotrophs. These data suggest that a background sodium current is essential to drive the membrane to threshold for firing spontaneous calcium-dependent action potentials in lactotrophs. This, in turn, results in elevated intracellular calcium, which supports spontaneous secretion of prolactin from these cells.

The effect of acetylcholine on inositol lipid metabolism and intracellular calcium concentrations in bovine anterior pituitary cells

Acetylcholine (ACh) increased the intracellular calcium concentration in bovine anterior pituitary cells. In the presence of the calcium channel antagonists verapamil (20 microM) or nitrendepine (1 microM) the increase in calcium was partially inhibited but showed both transient and sustained components. In the presence of EGTA (2.5 mM) only the transient component was observed. ACh also decreased inositol radioactivity in phosphatidylinositides and increased it in inositol phosphates. It is concluded that the increase in calcium caused by acetylcholine requires both the entry of external calcium and mobilisation of internal calcium. Replacement of external sodium by N-methyl-D-glucamine inhibited the rises in calcium and inositol phosphate labelling in response to ACh. Tetrodotoxin (3 microM) or ouabain (50 microM) did not affect either response to ACh. Verapamil did not affect the calcium rise induced by ACh in the absence of external sodium. The phorbol ester PMA (10 nM) caused a transient rise in calcium and inhibited the calcium rise caused by acetylcholine: it did not modify the effect of acetylcholine on inositol phosphates. The dependence of the stimulation of external calcium entry and inositol phosphate production on external sodium ions and protein kinase C is discussed.

Ion permeation and rectification in ATP-sensitive channels from insulin-secreting cells (RINm5F): effects of K+, Na+ and Mg2+

Patch-clamp techniques were used to study the permeability to ions of an ATP-sensitive channel in membranes from the pancreatic B-cell line (RINm5F). With patches in the outside-out configuration, the I-V curves for different Na+-K+ mixtures in the bath and 140 mM K+ in the pipette were almost linear, and crossed the zero-current axis at voltages that indicated a variable permeability ratio. When K+ was added symmetrically, the plot of the conductance vs. K+ activity exhibited saturation, with a Gmax of about 160 pS and a half-maximal activity of 216 mM. The I-V behavior for different K+-Na+ mixtures in the bath could be accurately described with a model based on Eyring theory, assuming two sites and one-ion occupancy. For K+, the dissociation constants (KK) of the two sites were 290 and 850 mM, the lower value pertaining to the site close to the intracellular medium. In experiments with inside-out patches, both Na+ and Mg2+, when present in the bath, induced a voltage-dependent block of the outward current. Fitting the data with the model suggested that for these ions only one of the two sites binds significantly, the corresponding dissociation constants being (mM): 46 for Na+ and 34 for Mg2+. Blocking by Na+ and Mg2+ may account for the low outward current seen in intact cells. This hypothesis is consistent with the observation that such current is further reduced by addition of 2,4-DNP, since metabolism inhibitors are expected to lower the ATP level, thereby liberating Mg2+ from the Mg2+-ATP complex, as well as inducing accumulation of Na+ by decreasing the rate of the Na+-K+ pump.

Sodium and potassium currents involved in action potential propagation in normal bovine lactotrophs

1. The properties of whole-cell and single-channel Na+ and K+ currents in immunocytochemically identified bovine lactotrophs were studied using the patch-clamp technique. 2. In the whole-cell, current-clamp mode, cells had membrane potentials of -94.7 +/- 6.7 mV and input resistances of 2-17 G omega. Current-induced action potentials were recorded with a threshold around -35 mV and amplitude of 40-65 mV. Repetitive firing was not sustained at frequencies greater than 1-2 Hz without total inactivation. 3. Under voltage clamp, action potentials were shown to be composed of an inward TTX-sensitive Na+ current and an outward K+ current that was abolished by internal Cs+. 4. The isolated Na+ current had a threshold for activation around -35 mV and rapidly inactivated to a steady state during a test voltage pulse. Inactivation was strongly voltage-dependent, with the Na+ current being half-inactivated at -20 mV. 5. Recovery from inactivation was voltage dependent and at a holding potential of -60 mV, 50% reactivation was achieved after 420 ms. The implications of this long reactivation time on sustained action potential frequency are discussed. 6. Single Na+ channel activity was examined with the outside-out patch configuration and yielded single-channel conductances of 22.5 pS. Reconstruction of the voltage and time dependence of single-channel currents provided an accurate picture of the whole-cell Na+ current. 7. Whole-cell outward current carried by K+ in the absence of Na+ and Ca2+ had a large conductance, was slowly activated and demonstrated no inactivation. A second, more rapidly activating Ca2+-dependent K+ current could also be demonstrated. 8. Ensemble analysis of whole-cell K+ currents in the absence of Ca2+ showed underlying single-channel amplitudes of 1.2 pA at +10 mV, with the lactotroph having about 350 active channels at this potential. 9. Recordings of single K+ channels also demonstrated two classes of channel: a small (50 pS) voltage-activated channel and a higher-conductance (100 pS) Ca2+- sensitive channel. 10. Prolactin secretion was shown to be TTX-insensitive but sensitive to membrane potential, demonstrated as increased release following increased external K+ but not Na+ concentration.(ABSTRACT TRUNCATED AT 400 WORDS)