Blockers of potassium current and resting membrane potential in rat muscle fibers.

Rat diaphragm fibers were equilibrated for several hours in 150 mM KCl; when they were returned to 5 mM KCl the resting potential went back to its original level with a half time of 17 min. This repolarization was blocked by 5 mM BaCl2, a blocker of the inward rectifier K channel. On the other hand, 0.1 mM apamin and 0.02 mM glibenclamide which block the Ca-dependent and ATP sensitive K channels, respectively, and 0.1 mM 9-AC a blocker of the Cl- channel did not affect the repolarization. 5 mM barium decreased the K conductance measured under current-clamp conditions in diaphragm muscle fibers. The possible role of the inward rectifier system in the repolarization following return to normal [K]o is discussed.     

Facilitation of spontaneous acetylcholine release induced by activation of cAMP in rat neuromuscular junctions

Regulation of neurotransmitter release is thought to involve modulation of the release probability by protein phosphorylation. Activation of the cAMP-protein kinase A (PKA) pathway has been shown to facilitate synaptic transmission in mammalian neuromuscular synapses, although the relevant phosphorylation targets are mostly unknown. We found that the inhibitor of the phosphodiesterase aminophylline (1 mM AMIN), the membrane-permeable analog of cAMP, 8-Br-cAMP (5 mM) and, the direct adenylate cyclase activator, forskolin (20 microM), induced an increase of miniature end-plate potentials (MEPPs) frequency in rat neuromuscular junctions. We investigated the possible involvement of the voltage-dependent calcium channels (VDCC), since these proteins are known to be phosphorylated by PKA. But this possibility was ruled out, since the increase in MEPPs frequency was not attenuated by the VDCC blocker Cd2+ (100 microM) and it was observed when AMIN was studied on hyperosmotic response, which is independent of [Ca2+]o and of Ca2+ influx through the VDCC. The lack of action of AMIN on MEPPs frequency when [Ca2+]i was diminished by exposing the preparations to zero Ca2+-EGTA solution (isotonic condition) or when nerve terminals were loaded with a permeant Ca2+ chelator (BAPTA-AM) (hypertonic condition), indicate that cAMP-mediated presynaptic facilitation is a function of nerve terminal Ca2+ concentration. We also found that AMIN exerted a comparable increase in MEPPs frequency in control and high K+ (10 and 15 mM), suggesting a single mechanism of action for spontaneous and K+-induced secretion.     

Spontaneous acetylcholine release in mammalian neuromuscular junctions

Spontaneous secretion of the neurotransmitter acetylcholine inmammalian neuromuscular synapsis depends on theCa²⁺ content of nerve terminals.The Ca²⁺ electrochemical gradientfavors the entry of this cation. We investigated the possibleinvolvement of three voltage-dependent Ca²⁺ channels (VDCC) (L-, N-, andP/Q-types) on spontaneous transmitter release at the rat neuromuscularjunction. Miniature end-plate potential (MEPP) frequency was clearlyreduced by 5 µM nifedipine, a blocker of the L-type VDCC, and to alesser extent by the N-type VDCC blocker, -conotoxin GVIA (-CgTx,5 µM). On the other hand, nifedipine and -CgTx had no effect onK⁺-induced transmitter secretion.-Agatoxin IVA (100 nM), a P/Q-type VDCC blocker, preventsacetylcholine release induced byK⁺ depolarization but failed toaffect MEPP frequency in basal conditions. These results suggest thatin the mammalian neuromuscular junction Ca²⁺ enters nerve terminalsthrough at least three different channels, two of them (L- and N-types)mainly related to spontaneous acetylcholine release and the other(P/Q-type) mostly involved in depolarization-induced neurotransmitterrelease. Ca²⁺-bindingmolecule-related spontaneous release apparently binds Ca²⁺ very rapidly and wouldprobably be located very close toCa²⁺ channels, since the fastCa²⁺ chelator (BAPTA-AM)significantly reduced MEPP frequency, whereas EGTA-AM, exhibitingslower kinetics, had a lower effect. The increase in MEPP frequencyinduced by exposing the preparation to hypertonic solutions wasaffected by neither external Ca²⁺concentration nor L-, N-, and P/Q-type VDCC blockers, indicating thatextracellular Ca²⁺ is notnecessary to produce hyperosmotic neurosecretion. On the other hand,MEPP frequency was diminished by BAPTA-AM and EGTA-AM to the sameextent, supporting the view that hypertonic response is promoted by"bulk" intracellular Ca²⁺concentration increases.

     

Maternal asthma,asthma medication use,and the risk of congenital heart defects

BACKGROUND: Asthma is a common problem that complicates pregnancy. Several drugs are considered acceptable for use during pregnancy, although none have been classified as safe. Few studies have assessed the health impact of maternal asthma/medication use on the fetus. METHODS: A population‐based case‐control study was conducted in New York State to determine if cardiac congenital malformations in offspring were associated with maternal use of asthma medication and/or maternal asthma. Cases were cardiac anomalies in the New York State Congenital Malformations Registry. Controls were live births without any major birth defects randomly selected from birth certificates and frequency matched by year of birth. Data were collected through a 30 min telephone interview. Exposure was maternal asthma/medication use, maternal asthma/no medication use, no asthma/medication use, and no asthma/no medication use (reference). RESULTS: A total of 502 (59.4%) cases and 1,066 (53.8%) controls participated. A positive association was seen between any heart defect and women with asthma who used medication (OR 2.38; 95% CI: 1.18, 4.82). No significant associations were observed between heart defects and either women with asthma who did not use medication or women without asthma who used asthma medications. When considering types of medication used, offspring of women with asthma who used bronchodilators had an increased risk of any heart defect (OR 2.20; 95%CI: 1.05, 4.61). CONCLUSIONS: These results suggest that both maternal asthma status (controlled vs. uncontrolled; severe vs. mild) and asthma medication use, particularly bronchodilators, may play a role in cardiac malformations in offspring. Birth Defects Research (Part A), 2009. © 2008 Wiley‐Liss, Inc.     

Effect of aminophylline on the contraction threshold of rat diaphragm fibers.

We studied the effect of aminophylline (0.1-1 mM) on the contraction threshold (CT) of rat diaphragm fibers (25 degrees C). The CT was measured by direct visualization (x200) of the fiber under current-clamp conditions. The main findings are the following: 1) Aminophylline lowers the CT, in a dose-dependent manner, toward more negative values of the resting membrane potential (Vm). 2) Dibutyryl adenosine 3',5'-cyclic monophosphate (2 mM) shifts the CT, although this change is smaller than in the presence of xanthine. 3) Tetracaine (1 mM), a drug that diminishes Ca release from the sarcoplasmic reticulum, reduces the shift induced by 1 mM aminophylline; this is partially overcome by increasing aminophylline concentration to 5 mM. 4) Hyperpolarization of the fibers shifts the CT to more negative Vm. We suggest that the displacement in the CT to more negative Vm plays an important role in the potentiating effect of aminophylline. This could be the result of an enhancement of Ca release from the sarcoplasmic reticulum.