Ca2+ imaging in perfused adrenal medullae.

The Ca2+ imaging method was developed to explore changes in excitability in adrenal medullary (AM) cells in a large field in response to synaptic input and chemicals. The adrenal medullae of rats and guinea pigs were retrogradely loaded with Ca2+ indicator through the adrenal vein. Nerve fibers remaining in the adrenal gland were electrically stimulated to induce postsynaptic responses in AM cells, and chemicals were applied to the cells by adding to the perfusate. With this method, gamma-aminobutyric acid (GABA) was shown to increase the Ca2+ signal in almost all and 40% AM cells in guinea pigs and rats, respectively.

Mechanisms and roles of muscarinic activation in guinea-pig adrenal medullary cells.

Muscarinic receptors are expressed in the adrenal medullary (AM) cells of various mammals, but their physiological roles are controversial. Therefore, the ionic mechanism for muscarinic receptor-mediated depolarization and the role of muscarinic receptors in neuronal transmission were investigated in dissociated guinea-pig AM cells and in the perfused guinea-pig adrenal gland. Bath application of muscarine induced an inward current at -60 mV. This inward current was partially suppressed by quinine with an IC(50) of 6.1 µM. The quinine-insensitive component of muscarine-induced currents changed the polarity at -78 mV and was inhibited by bupivacaine, a TWIK-related acid-sensitive K(+) (TASK) channel inhibitor. Conversely, the current-voltage relationship for the bupivacaine-insensitive component of muscarine currents showed a reversal potential of -5 mV and a negative slope below -40 mV. External application of La(3+) had a double action on muscarine currents of both enhancement and suppression. Immunoblotting and immunocytochemistry revealed expression of TASK1 channels and cononical transient receptor potential channels 1, 4, 5, and 7 in guinea-pig AM cells. Retrograde application of atropine reversibly suppressed transsynaptically evoked catecholamine secretion from the adrenal gland. The results indicate that muscarinic receptor stimulation in guinea-pig AM cells induces depolarization through inhibition of TASK channels and activation of nonselective cation channels and that muscarinic receptors are involved in neuronal transmission from the splanchnic nerve.

Identification and role of muscarinic receptor subtypes expressed in rat adrenal medullary cells.

The muscarinic receptor is known to be involved in the acetylcholine (ACh)-induced secretion of catecholamines in the adrenal medullary (AM) cells of various mammals. The muscarinic receptor subtype involved and its physiological role, however, have not been elucidated yet. Thus, we investigated these issues in acutely isolated rat AM cells and perfused rat adrenal medulla. The RT-PCR analysis revealed the presence of M(2), M(3), M(4), and M(5) mRNAs. Immunocytochemistry with specific antibodies showed that M(5)-like immunoreactivities (IRs) were detected at half the cell membrane area, which was much larger than that with M(3)- or M(4)-like IRs. Muscarine produced inward currents in a dose-dependent manner. Pilocarpine, McN-A-343, and oxotremorine were less efficient than muscarine; and RS-86, which has no action on the M(5) receptor, produced no current. Electrical stimulation of nerve fibers produced a frequency-dependent increase in the Ca(2+) signal in perfused adrenal medullae. Muscarinic receptors were found to be involved in neuronal transmission in AM cells in the presence of a cholinesterase inhibitor, which suppresses ACh degradation. We concluded that the M(5) receptor is the major muscarinic receptor subtype in rat AM cells and may be involved in neuronal transmission under conditions where ACh spills over the synapse.

Paracrine role of GABA in adrenal chromaffin cells.

The function of GABA in the adrenal medulla is still controversial. We will review experimental results in vivo and in vitro in adrenal chromaffin cells of various mammals to clarify what has been elucidated and what still remains to be settled.

Ca2+ pathway involved in the refilling of store sites in rat adrenal medullary cells.

It has been suggested that store-operated Ca(2+) entry (SOC) facilitates catecholamine secretion and synthesis in bovine adrenal medullary (AM) cells. However, there has been no experimental result clearly showing that cation channel activity is enhanced by store Ca(2+) depletion. Thus the present experiments were undertaken to address the issue of whether rat AM cells have SOC channels. Inhibition of the sarco(endo)plasmic reticulum Ca(2+) (SERCA) pump resulted in a sustained increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in rat AM cells. This increase was completely suppressed by 2 mM Ni(2+) but not by 100 muM D600. A bath application of Ni(2+), but not D600, produced an outward current at -60 mV in rat AM cells, whereas exposure to a SERCA pump inhibitor did not affect either the whole cell current level or the Ni(2+)-induced outward current. The refilling of intracellular store sites was suppressed by the addition of Ni(2+) to the perfusate. RT-PCR revealed that transcripts for transient receptor potential channels 1 (TRPC1) and 5 (TRPC5) were present in rat adrenal medullas. Immunocytochemistry showed that TRPC1 channels, which have been implicated in SOC in certain types of cells, were mainly localized in the endoplasmic reticulum (ER) and not in the plasma membrane, and that STIM1, a Ca(2+) sensor in the ER, was not expressed in rat AM cells. On the basis of these results, we conclude that rat AM cells lack the SOC mechanism.

Molecular mechanisms supporting a paracrine role of GABA in rat adrenal medullary cells.

GABA is known to produce membrane depolarization and secretion in adrenal medullary (AM) cells in various species. However, whether the GABAergic system is intrinsic or extrinsic or both in the adrenal medulla and the role that GABA plays are controversial. Therefore, these issues were addressed by combining a biochemical and functional analysis. Glutamic acid decarboxylase (GAD), a GABA synthesizing enzyme, and vesicular GABA transporter (VGAT) were expressed in rat AM cells at the mRNA and protein levels, and the adrenal medulla had no nerve fibre-like structures immunoreactive to an anti-GAD Ab. The double staining for VGAT and chromogranin A indicates that GABA was stored in chromaffin granules. The alpha1, alpha3, beta2/3, gamma2 and delta subunits of GABA(A) receptors were identified in AM cells at the mRNA and protein levels. Pharmacological properties of GABA-induced Cl(-) currents, immunoprecipitation experiments and immunocytochemistry indicated the expression of not only gamma2-, but also delta-containing GABA(A) receptors, which have higher affinities for GABA and neurosteroids. Expression of GATs, which are involved in the clearance of GABA at GABAergic synapses, were conspicuously suppressed in the adrenal medulla, compared with expression levels of GABA(A) receptors. Increases in Ca(2+) signal in AM cells evoked trans-synaptically by nerve stimulation were suppressed during the response to GABA, and this suppression was attributed to the shunt effect of the GABA-induced increase in conductance. Overall Ca(2+) responses to electrical stimulation and GABA in AM cells were larger or smaller than those to electrical stimulation alone, depending on the frequency of stimulation. The results indicate that GABA functions as a paracrine in rat AM cells and this function may be supported by the suppression of GAT expression and the expression of not only gamma2-, but also delta-GABA(A) receptors.

Inhibition of TASK1-like channels by muscarinic receptor stimulation in rat adrenal medullary cells.

The muscarinic receptor is known to be involved in the acetylcholine-induced secretion of catecholamines in the adrenal medulla (AM) cells of various mammals. The ionic mechanisms, however, have not been elucidated yet. Thus, we investigated the issue in acutely isolated rat AM cells with the perforated patch clamp method. Bath application of 30 muM muscarine induced depolarization with the consequent generation of action potentials or an inward current at negative membrane potentials. The muscarine-sensitive current instantaneously changed in amplitude upon application of command pulses without a time-dependent component, altered the polarity as a K(+)-electrode, and showed rectification of the Goldman-Hodgkin-Katz (GHK) type. The whole-cell current at -20 mV was inhibited by external H(+) ions with a concentration responsible for half inhibition of pH 7.09 and muscarine failed to induce a further inward current during exposure to a saline in which pH decreased to 6.5. A similar occlusion occurred in secretion when pH in muscarine-containing saline decreased to 6.6. RT-PCR, immunoblotting, and immunocytochemistry suggested that rat AM cells mainly express the TASK1 channel. This TASK channel in AM cells may directly sense a decrease in blood pH, which occurs during exercise. The muscarine action was mimicked by oxotremorine-methiodide, but not by oxotremorine. The present results indicate that activation of muscarinic receptors or a decrease in external pH in the rat AM cell induces secretion through the inhibition of TASK1-like channels.

Mode of mitochondrial Ca2+ clearance and its influence on secretory responses in stimulated chromaffin cells.

To study the role of mitochondrial Ca(2+) clearance in stimulated cells, changes in free Ca(2+) concentration in the cytosol, [Ca(2+)](c) and that in mitochondria, [Ca(2+)](m) along with secretory responses were observed using chromaffin cells co-loaded with Fura-2 and Rhod-2 in the perfused rat adrenal medulla. When the cells were stimulated with 40 mM K(+) in the perfusate, the duration of [Ca(2+)](m) response markedly increased with prolongation of the stimulation period, exhibiting a mean half-decay time of 21 min with 30s stimulation, whereas its amplitude was not altered with stimulations of 10-30s. A computer simulation analysis showed that such a mode of [Ca(2+)](m) response can be produced if excess Ca(2+) taken up by mitochondria precipitates as calcium phosphate (Pi) salt. In the presence of 5 microM rotenone plus 10 microM oligomycin, a decrease in the duration of [Ca(2+)](m) response and a slight but significant increase (24%) in the secretory response to 30s stimulation with 40 mM K(+) were observed. Simulation analyses suggested that this effect of rotenone may be due to reduction in mitochondrial Ca(2+) uptake induced by rotenone-elicited partial depolarization of the mitochondrial membrane potential. In chromaffin cells transsynaptically stimulated through the splanchnic nerve, the intensity of NAD(P)H autofluorescence changed with time courses similar to those of [Ca(2+)](m) responses. The temporal profiles of those two responses were prolonged in a similar manner by application of an inhibitor of mitochondrial Na(+)/Ca(2+) exchanger, CGP37157. Thus, due to the unique Ca(2+) buffering mechanism, [Ca(2+)](m) responses associated with massive mitochondrial Ca(2+) uptake may occur within a limited concentration range in which Ca(2+)-sensitive dehydrogenases are activated to control the mitochondrial redox state in stimulated chromaffin cells.

Mechanisms for hypoxia detection in O2-sensitive cells.

Since O(2) is the bare necessity for multicellular organisms, they develop multiple protective mechanisms against hypoxia. Mammals will adapt to hypoxia in short and long terms. The short-term responses include enhancement of the respiratory and cardiac functions, adrenaline secretion from adrenal medullary cells, and pulmonary vasoconstriction, whereas the long-term response is the increase in erythropoietin production with the consequent increase in red blood cells. Although much work has been done to elucidate molecular mechanisms for O(2)-sensing for the last ten years, the majority of the mechanisms remain unclear. We will review mechanisms proposed for hypoxia detection in carotid body type I cells, pulmonary artery smooth muscle, adrenal medullary cells, and liver cells, with the special focus on adrenal medullary cells.

InsP3 receptor type 2 and oscillatory and monophasic Ca2+ transients in rat adrenal chromaffin cells.

Muscarinic receptor stimulation induced oscillatory and monophasic Ca(2+) transients in rat adrenal chromaffin cells in the absence of external Ca(2+). As this Ca(2+) mobilization may be mediated by InsP(3), we first explored types of InsP(3) receptors and their intracellular distribution in chromaffin cells. The InsP(3) receptor type 1 was not immunodetected in precipitates of adrenal medulla homogenates and in dissociated adrenal chromaffin cells, whereas an anti-type 3 mAb recognized a faint band with about 250 kDa, but no significant immunoreaction was visible in chromaffin cells. The anti-type 2 mAb strongly detected a band with about 220 kDa and the immunoreaction was observed perinuclearly and at the cell periphery. These results indicate that InsP(3) receptor type 2 is predominant in chromaffin cells. The oscillatory and monophasic Ca(2+) transients were reproduced in simulation based on a three-state kinetic model (shut, open, and inactivated states). Ca(2+) ions were found experimentally and theoretically to turn over rapidly between stores and the cytosol during stimulation. The results suggest that InsP(3) receptor type 2 is responsible for both oscillatory and monophasic Ca(2+) transients and that change in mode of Ca(2+) responses may be accounted for by the kinetic property of the type 2 receptor.

Homogeneous Ca2+ stores in rat adrenal chromaffin cells.

The localization and function of Ca(2+) stores in isolated chromaffin cells of rat adrenal medulla were investigated using confocal laser microscopy and amperometry. Binding sites for BODIPY-inositol 1,4,5-trisphosphate (IP(3)), -ryanodine (Ry), and -thapsigargin (Thap) were both perinuclear and at the cell periphery. The endoplasmic reticulum (ER), which was identified by ER Tracker dye, took up fluorescent Ry and IP(3), and the majority of BODIPY-Ry-binding area was bound by fluorescent IP(3). Under Ca(2+)-free conditions, the amount of caffeine-induced catecholamine secretion was 33% of that of muscarine-induced secretion, but muscarine induced little or no secretion after exposure to caffeine. Muscarine-induced Ca(2+) increases, as observed with fluo-3, lasted for a few tens of seconds under Ca(2+)-free conditions, whereas a caffeine-induced Ca(2+) transient diminished rapidly with a half decay time of 3s and this spike-like Ca(2+) transient was then followed by a sustained increase with a low level. These results indicate that IP(3) receptors and Ry receptors (RyRs) are present in common ER Ca(2+) storage and the lower potency of caffeine for secretion may be due to a rapid decrease in RyR channel activity to a low level.