Effects of an application of granular carbaryl on nontarget forest floor arthropods.

We evaluated the effects of a single application of granular carbaryl made against nymphal Ixodes scapularis Say on the diversity and abundance of forest arthropods taken in pitfall traps in oak and mixed oak-pine forest sites for 12 wk after treatment in central New Jersey. Significant short-term changes in arthropod assemblages were detected immediately posttreatment. Effects were not distributed equally across taxa. Seasonal changes in numbers and diversity of forest arthropods in the study areas may have affected the impact of the acaricide in the treatment area. Comparison with control areas indicated that reductions in abundance of some arthropod taxa in the treatment area were detectable 12 wk after treatment.

Effects of flecainide on contractile force and electrophysiological parameters in cardiac muscle.

The action of N-(2-piperidylmethyl)-2,5-bis-(2,2, 2-trifluoroethoxy)-benzamide acetate (flecainide, Tambocor) on contractile force and electrophysiological parameters was studied in isolated calf ventricular fibres. Increasing flecainide concentrations (2-10 mg/l) induced a progressive decrease of force of contraction. At the highest concentration the peak of contraction was reduced by 50-40% of the control value, accompanied by a similar fall of the maximal rate of the rise of contraction (dP/dtmax). The negative inotropic effect was not significantly different at driving frequencies of 0.2 and 1 Hz. 5 mg/l flecainide reduced the tonic as well as the phasic components in high potassium contractures and induced a reduction of the action potential maximal upstroke velocity (Vmax). The suppression of Vmax was augmented by increasing the stimulation frequency from 0.2 to 1 Hz. This frequency dependence could partially be explained by a flecainide induced delay of Vmax recovery from inactivation. Plateau height and duration of the action potential were not significantly effected. The height and duration of slow potentials in 22 mmol/l K+-Tyrode solution were reduced under 5 mg/l flecainide. This effect was frequency independent, since the recovery of the slow potentials from inactivation was not delayed by flecainide. The results indicate that the new antiarrhythmic drug flecainide possesses two different sites of action, indicated by the different frequency dependence on Vmax and slow potential system under flecainide. The observed negative inotropic effect seems to be induced by the depressing effect of flecainide on the slow potential system.

Observations on the staircase phenomenon in guinea pig atrium.

The staircase phenomenon, occurring after a change in frequency was studied in isolated trabeculae from guinea pig atrium. The effects on tension, action potential form and function as well as ionic currents were investigated. 1. In the ascending part of the frequency-force relationship a sudden change to a new driving frequency resulted in a staircase response which consisted of two exponential phases (tau 1 = 1-2 s; tau 2 = 20-30 s). 2. The build-up or decline of twitch tension in response to either an increase or reduction of [Ca2+]0 followed a similar composed time course. 3. After a reduction in stimulation frequency the action potentials changed time dependently: In the first response the peak of the overshoot was reached faster and the plateau phase was shortened; afterwards these parameters remained constant while the repolarization phase continually shortened during the following 5-10 action potentials. 4. In voltage clamp experiments an analogous reduction in the frequency of depolarizing voltage clamp pulses induced an immediate increase of the slow inward current (Isi). In the following 10 pulses the increased Isi remained constant, while the late outward current continually increased. 5. The time course of recovery of the Isi-system was found to be slow in atrial trabeculae (tau = 300-500 ms at -70 mV). Thus the increase in Isi, observed after a reduction in frequency, could be explained by a more complete recovery of the Isi-system during the interval between two stimuli. 6. the increase in Isi during these recovery experiments was accompanied by an accelerated inactivation. 7. It is concluded that after a reduction in stimulation rate the faster development of the overshoot and the shortening of the plateau phase are due to an augmentation and a faster inactivation of the Isi, respectively. The shortening of the late repolarization phase which developed during successive action potentials is most probably related to the observed increase in late outward current.

The effects of beta-sympathomimetic amines and phosphodiesterase inhibitors on electrophysiological parameters in Purkinje fibres.

Dose-dependent responses to L-isoprenaline of spontaneous activity and plateau height in Purkinje fibres can be mimicked closely by the PDE-inhibitor IBMX. Simultaneous applications of sympathomimetic amines and IBMX result in potentiated responses. The results support the hypothesis that cyclic-AMP is the common mediator of positive chronotropic and inotropic effects induced by beta-sympathomimetic amines.

The effects of isoprenaline and a new beta-sympathomimetic amine upon spontaneous activity, diastolic depolarization and plateau height in cardiac Purkinje fibres.

1 In spontaneously active Purkinje fibres of young cows the dose-response curves of the action of isoprenaline upon different electrophysiological parameters were measured.2 The increase in slope of diastolic depolarization could roughly be described by a one-for-one binding curve with a half maximum effect near 10(-8) M and the increase in the height of the plateau level by a two-for-one binding curve with a half maximum effect near 10(-7) M (-)-isoprenaline.3 These dose-response curves were similar to those of two parameters measured under voltage clamp conditions by other authors. The increase in slope of diastolic depolarization behaved like the shift of the activation curve for the pacemaker potassium current towards positive potentials and the growth in plateau height like the increase in the slow inward current mainly carried by Ca ions. From this conformity we propose that the parameters evaluated by us from action potential records could be used for a qualitative analysis of the action of catecholamines on pacemaker potassium current and Ca influx.4 The effects of the isomers of a new drug, 1-isopropylamino-3(4'hydroxyphenoxy)-propan-2-ol (IHP), were evaluated in the same way as those of isoprenaline. The (-)-isomer was at optimal concentrations (10(-5) M) nearly half as effective as isoprenaline in increasing frequency and slope of diastolic depolarization but caused no increase in plateau height. An identical relationship, but at 5 to 10 times higher concentrations, was obtained with the (+)-isomer.5 When 10(-4) M(-)-IHP was added to a preparation equilibrated with a maximum dose of (-)-isoprenaline (10(-6) M), frequency and plateau height declined. This result together with the observation that the effects of IHP could be blocked by the specific beta-antagonist propranolol, revealed the beta-agonistic nature of the new drug. Its inefficiency in increasing the plateau height and thus the slow (Ca) inward current was explained by its relatively low potency and intrinsic activity.