Adenine nucleotides and intracellular Ca2+ regulate a voltage-dependent and glucose-sensitive potassium channel in neurosecretory cells

Large-conductance Ca2+-activated potassium channels in secretory neurons

Large-conductance Ca2+-activated K+ channels (BK) are believed to underlie interburst intervals and contribute to the control of hormone release in several secretory cells. In crustacean neurosecretory cells, Ca2+ entry associated with electrical activity could act as a modulator of membrane K+ conductance. Therefore we studied the contribution of BK channels to the macroscopic outward current in the X-organ of crayfish, and their participation in electrophysiological activity, as well as their sensitivity toward intracellular Ca2+, ATP, and voltage, by using the patch-clamp technique. The BK channels had a conductance of 223 pS and rectified inwardly in symmetrical K+. These channels were highly selective to K+ ions; potassium permeability (PK) value was 2.3 x 10(-13) cm(3) s(-1). The BK channels were sensitive to internal Ca2+ concentration, voltage dependent, and activated by intracellular MgATP. Voltage sensitivity (k) was approximately 13 mV, and the half-activation membrane potentials depended on the internal Ca2+ concentration. Calcium ions (0.3-3 microM) applied to the internal membrane surface caused an enhancement of the channel activity. This activation of BK channels by internal calcium had a KD(0) of 0.22 microM and was probably due to the binding of only one or two Ca2+ ions to the channel. Addition of MgATP (0.01-3 mM) to the internal solution increased steady state-open probability. The dissociation constant for MgATP (KD) was 119 microM, and the Hill coefficient (h) was 0.6, according to the Hill analysis. Ca2+-activated K+ currents recorded from whole cells were suppressed by either adding Cd2+ (0.4 mM) or removing Ca2+ ions from the external solution. TEA (1 mM) or charybdotoxin (100 nM) blocked these currents. Our results showed that both BK and K(ATP) channels are present in the same cell. Even when BK and K(ATP) channels were voltage dependent and modulated by internal Ca2+ and ATP, the profile of sensitivity was quite different for each kind of channel. It is tempting to suggest that BK and KATP channels contribute independently to the regulation of spontaneous discharge patterns in crayfish neurosecretory cells.

P-type Ca2+ current in crayfish peptidergic neurones

Inward Ca2+ current through voltage-gated Ca2+ channels was recorded from freshly dissociated crayfish X-organ (XO) neurones using the whole-cell voltage-clamp technique. Changing the holding potential from −50 to −90 mV had little effect on the characteristics of the current-voltage relationship: neither the time course nor the amplitude of the Ca2+ current was affected. Inactivation of the Ca2+ current was observed over a small voltage range, between −35 and −10 mV, with half-inactivation at −20 mV. The activation of the Ca2+ current was modelled using Hodgkin-Huxley kinetics. The time constant of activation, τ m, was 568+/−66 micros at −20 mV and decreased gradually to 171+/−23 micros at 40 mV (means +/− s.e.m., N=5). The steady-state activation, m(infinity), was fitted with a Boltzmann function, with a half-activation voltage of −7.45 mV and an apparent threshold at −40 mV. The instantaneous current-voltage relationship was adjusted using the Goldman-Hodgkin-Katz constant-field equation, giving a permeation of 4.95×10(−5)cm s-1. The inactivation of the Ca2+ current in XO neurones was dependent on previous entry of Ca2+. Using a double-pulse protocol, the inactivation was fitted to a U-shaped curve with a maximal inactivation of 35 % at 30 mV. The time course of the recovery from inactivation was fitted with an exponential function. The time constants were 17+/−2.6 ms for a prepulse of 10 ms and 31+/−3.2 ms for a prepulse of 20 ms. The permeability sequence of the Ca2+ channels was as follows: Ba2+>Sr2+~Ca2+>>Mg2+. Other divalent cations blocked the Ca2+ current, and their effects were voltage-dependent; the potency of blockage was Cd2+~Zn2+>>Co2+~Ni2+. The peptide ω -agatoxin-IVA, a selective toxin for P-type Ca2+ channels, blocked 85 % of the Ca2+ current in XO neurones at 200 nmol l-1, but the current was insensitive to dihydropyridines, phenylalkylamines, ω -conotoxin-GVIA and ω -conotoxin-MVIIC, which are blockers of L-, N- and Q-type Ca2+ channels, respectively. From the voltage- and Ca2+-dependent kinetics, the higher permeability to Ba2+ than to Ca2+ and the higher sensitivity of the current to Cd2+ than to Ni2+, we conclude that the Ca2+ current in XO neurones is generated by high-voltage-activated (HVA) channels. Furthermore, its blockage by ω -agatoxin-IVA suggests that it is mainly generated through P-type Ca2+ channels.

Regulation of crustacean neurosecretory cell activity

1. The X organ-sinus gland system is a conglomerate of 150-200 neurosecretory cells in the eyestalk of crustaceans. It is the source of a host of peptide neurohormones which partake in the control of a wide range of physiological functions. Distinct families of X organ peptides have been chemically characterized: (a) two chromatophorotropic hormones of small sizes, one of 8 residues and the other of 15-20 residues; and (b) three metabotropic hormones of high molecular weight (70-80 residues), related to the control of blood sugar levels, molting, and gonad activity. Some of these hormones have been identified only in crustaceans; others are common to various arthropod groups. A number of peptides orginally described in other zoological groups are also present in the X organ-sinus gland system; such is the case for members of the FMRF-amide family, enkephalins, and other peptides. 2. Cells specifically containing each hormone have been located in the X organ and some information is available on the cellular and molecular substrate of the biosynthesis, transport, storage, and release of various hormones. The electrical activity of X organ neurons has been recorded at the cell soma, arborizations, axons, and neurosecretory terminals. Conspicuous regional differences have been defined for the various patterns of activity, as well as the distribution of their underlying ion currents. 3. The release of hormones and the electrical activity of X organ neurons are regulated by environmental and endogenous influences, such as light and darkness, stress, and circadian rhythms. These influences appear to be mediated by a host of neurotransmitters/modulators, most noticeably, gamma-aminobutyric acid, 5-hydroxytryptamine and other amines, and enkephalins. Each of these mediators acts upon a definite ionic substrate(s) and exerts specific regulatory effects on X organ cell activity. A given neuron may be under the control of more than one neurotransmitter, and a transmitter may mediate different and even opposite influences on different neurons.