Peter J. Dosen

Role of calcium stores and membrane voltage in the generation of slow wave action potentials in guinea‐pig gastric pylorus

1 Intracellular recordings made in single bundle strips of a visceral smooth muscle revealed rhythmic spontaneous membrane depolarizations termed slow waves (SWs). These exhibited ‘pacemaker’ and ‘regenerative’ components composed of summations of more elementary events termed spontaneous transient depolarizations (STDs). 2 STDs and SWs persisted in the presence of tetrodotoxin, nifedipine and ryanodine, and upon brief exposure to Ca2+‐free Cd2+‐containing solutions; they were enhanced by ACh and blocked by BAPTA AM, cyclopiazonic acid and caffeine. SWs were also inhibited in heparin‐loaded strips. 3 SWs were observed over a wide range of membrane potentials (e.g. ‐80 to ‐45 mV) with increased frequencies at more depolarized potentials. 4 Regular spontaneous SW activity in this preparation began after 1–3 h superfusion of the tissue with physiological saline following the dissection procedure. Membrane depolarization applied before the onset of this activity induced bursts of STD‐like events (termed the ‘initial’ response) which, when larger than threshold levels initiated regenerative responses. The combined initial‐regenerative waveform was termed the SW‐like action potential. 5 Voltage‐induced responses exhibited large variable latencies (typical range 0.3‐4 s), refractory periods of ≈11 s and a pharmacology that was indistinguishable from those of STDs and spontaneous SWs. 6 The data indicate that SWs arise through more elementary inositol 1,4,5‐trisphosphate (IP3) receptor‐induced Ca2+ release events which rhythmically synchronize to trigger regenerative Ca2+ release and induce inward current across the plasmalemma. The finding that action potentials, which were indistinguishable from SWs, could be evoked by depolarization suggests that membrane potential modulates IP3 production. Voltage feedback on intracellular IP3‐sensitive Ca2+ release is likely to have a major influence on the generation and propagation of SWs.

The Effect Of Salt Intake On Cyclosporine-induced Impairment Of Renal Function In Rats

Three groups of rats were fed a low-sodium diet. Groups 1 drank water and was treated with cyclosporine, 100 mg kg-1 48 h-1 p.o. for 3 weeks (low-salt-treated group). Group 2 drank 0.15 M saline and was also treated with cyclosporine (high-salt-treated group). Group 3 drank water and was treated with the vehicle (low-salt-vehicle group). Measurements were made during a control period and weekly during the 3-week treatment period and a 3-week recovery period. Both cyclosporine-treated groups lost weight during treatment but the rises in serum creatinine and blood urea and decrease in creatinine clearance were greater in the low-salt group. The vehicle group gained weight and had no change in the other parameters over the three weeks. There was an increase in urine volume and sodium excretion in the high-salt group associated with cyclosporine treatment. Although the low-salt groups had a higher plasma renin concentration than the high-salt group there were no changes produced by cyclosporine treatment. Histopathological examination showed mild tubular lesions with vacuolar degeneration of proximal tubular cells. This was more prevalent in the low-salt-cyclosporine-treated group. The plasma concentrations of cyclosporine were not different after one-week treatment but were slightly greater after 2 week and 3-week treatment in the low-salt group. We have suggested that the greater impairment of renal function in the low-salt group produced by cyclosporine may be contributed to by an involvement of tubuloglomerular feedback.

A pharmacological approach to first aid treatment for snakebite.

Snake venom toxins first transit the lymphatic system before entering the bloodstream. Ointment containing a nitric oxide donor, which impedes the intrinsic lymphatic pump, prolonged lymph transit time in rats and humans and also increased rat survival time after injection of venom. This pharmacological approach should give snakebite victims more time to obtain medical care and antivenom treatment.

Prevention of high blood pressure by reducing sympathetic innervation in the spontaneously hypertensive rat

It has previously been reported that the increase in blood pressure in the spontaneously hypertensive rat (SHR) occurs concurrently with a marked increase in thickness of the arterial wall and an increase in vascular innervation, particularly for the small muscular arteries. The purpose of the present study was to determine whether prevention of the increase in vascular innervation could prevent elevation of blood pressure in the SHR. We found that intraperitoneal injection of a single dose of an antiserum to nerve growth factor (anti-NGF) into young SHRs (postnatal day 19-24) caused a marked reduction in mean blood pressure at age 3-4 months from the raised value of 24.2 +/- 0.5 kPa to 18.9 +/- 0.8 kPa. By comparison, treated Wistar Kyoto rats (WKYs) maintained normal blood pressures. The treatment reduced the amplitude of the intracellularly recorded excitatory junction potential and the NA content of mesenteric arteries in the SHR, leaving the values similar to those of control WKYs. The NA content of these vessels was also reduced in treated WKYs. Importantly, the thickness of the vessel wall, which was greater in the SHR than the WKY, was not significantly altered by anti-NGF treatment. It is concluded that anti-NGF treatment during late neonatal development inhibits the increase in the functional levels of vascular innervation observed in the SHR. Furthermore, this increase in the functional levels of vascular innervation is necessary for the development of hypertension in this rat strain.

Pharmacological approaches that slow lymphatic flow as a snakebite first aid

Background This study examines the use of topical pharmacological agents as a snakebite first aid where slowing venom reaching the circulation prevents systemic toxicity. It is based on the fact that toxin molecules in most snake venoms are large molecules and generally first enter and traverse the lymphatic system before accessing the circulation. It follows on from a previous study where it was shown that topical application of a nitric oxide donor slowed lymph flow to a similar extent in humans and rats as well as increased the time to respiratory arrest for subcutaneous injection of an elapid venom (Pseudonaja textilis, Ptx; Eastern brown snake) into the hind feet of anaesthetized rats. Methodology/Principal Findings The effects of topical application of the L-type Ca2+ channel antagonist nifedipine and the local anesthetic lignocaine in inhibiting lymph flow and protecting against envenomation was examined in an anaesthetized rat model. The agents significantly increased dye-measured lymph transit times by 500% and 390% compared to controls and increased the time to respiratory arrest to foot injection of a lethal dose of Ptx venom by 60% and 40% respectively. The study also examined the effect of Ptx venom dose over the lethal range of 0.4 to 1.5 mg/kg finding a negative linear relationship between increase in venom dose and time to respiratory arrest. Conclusions/Significance The findings suggest that a range of agents that inhibit lymphatic flow could potentially be used as an adjunct treatment to pressure bandaging with immobilization (PBI) in snakebite first aid. This is important given that PBI (a snakebite first aid recommended by the Australian National Health and Medical research Council) is often incorrectly applied. The use of a local anesthetic would have the added advantage of reducing pain.

ENDOTHELIUM-DEPENDENT INHIBITION OF SYMPATHETIC VASOCONSTRICTION BY FRUSEMIDE ADMINISTRATION TO RATS

1. Segments of the tail artery of the rat were cannulated at both ends and mounted in an organ bath filled with Krebs solution.

Omeprazole: effects on oxidative drug metabolism in the rat.

1. Omeprazole, a substituted benzimidazole and a potent gastric antisecretory drug has been tested for inhibition of microsomal drug oxidative function in the rat.

Single mechanically-gated cation channel currents can trigger action potentials in neocortical and hippocampal pyramidal neurons.

The mammalian brain is a mechanosensitive organ that responds to different mechanical forces ranging from intrinsic forces implicated in brain morphogenesis to extrinsic forces that can cause concussion and traumatic brain injury. However, little is known of the mechanosensors that transduce these forces. In this study we use cell-attached patch recording to measure single mechanically-gated (MG) channel currents and their affects on spike activity in identified neurons in neonatal mouse brain slices. We demonstrate that both neocortical and hippocampal pyramidal neurons express stretch-activated MG cation channels that are activated by suctions of ~25mm Hg, have a single channel conductance for inward current of 50-70pS and show weak selectivity for alkali metal cations (i.e., Na(+)<K(+)<Cs(+)). Significantly, single MG channel currents activated on the soma trigger spiking/action potentials in both neocortical and hippocampal pyramidal neurons. Not all neuron types studied here expressed MG channel currents. In particular, locus coeruleus and cerebellar Purkinje neurons showed no detectable MG channel activity. Moreover their robust rhythmic spike activity was resistant to mechanical modulation. Our observation that a single MG channel current can trigger spiking predicates the need for reassessment of the long held view that the impulse output of central neurons depends only upon their intrinsic voltage-gated channels and/or their integrated synaptic input.

Effects of catecholamines and diazepam in chloroquine poisoning in barbiturate anaesthetised rats

The recommended treatment of human chloroquine poisoning is diazepam and adrenaline but neither has been evaluated in controlled clinical trials. We investi gated whether diazepam provided any added benefit over barbiturate anaesthesia and whether the protective effect of catecholamines in chloroquine poisoning was mediated through alpha or beta receptor stimulation. Rats, anaesthetised with thiobutobarbitone had a continuous intravenous infusion of 3 mg/kg/min of chlor oquine. This caused a steady decline in pulse rate and blood pressure. When diazepam (3 mg/kg iv) was admi nistered 5 -10 min later, heart rates decreased at a faster rate (P=0.005), blood pressure was consistently lower (P=0.01) and there was a shorter time to arrhythmias and death (P < 0.05). Adrenergic agents were given by titration to attempt to maintain mean blood pressure > 75 mmHg. Compared with the phenylephrine (selective alpha agonist) group, the group treated with isoprenaline (selective beta agonist) had faster heart rates which decreased more slowly (P < 0.0001), higher blood pressure (P=0.005) and longer time to arrhythmias and death (P=0.005). Adrenaline and noradrenaline had intermedi ate effects. Thus beta agonist effect appears to explain the beneficial effects of adrenaline but alpha agonist activity may be harmful. This animal work suggests that a combination of barbiturate anaesthesia and isoprenaline may be better than the diazepam and adrenaline in combatting the effects of chloroquine.