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Biomedical subjects

K R Courtney

Publications and source records attributed to K R Courtney.

43 records · Page 3Linked to original sources
PubMed1978

Prostaglandins and pacemaker activity in isolated guinea pig SA node.

Prostaglandins PGE2, PGE1, PGF2alpha, and PGE1 substantially increase automaticity in SA-nodal, right atrial preparations excised from guinea pigs. This natural pacemaker tissue is sensitive to nanomolar doses of PG with, for example, 10(-8) M PGE2, increasing SA rate by about 20%. If these preparations are pretreated with 2 micrometer indomethacin, a blocker of endogenous prostaglandin synthesis, then spontaneous rate drops and subsequent rate increases due to PGE2 administration can be more easily demonstrated. Guinea pig pacemaker tissue differs from similar rabbit tissue not only in that it is directly responsive to PGE2, but also in that PGE2 does not depress the absolute response to transmural stimulation (adrenergically mediated rate increase). The positive chronotropic responses to PGE2 also occur when the guinea pig tissue is pretreated in 0.6 micrometer propranolol, which causes blockade of beta-adrenergic receptors. The pacemaker myocardium in the guinea pigs thus appears to be directly stimulated by exogenous PGE2 at very low doses. The observation that 2 micrometer indomethacin reduces SA-nodal rate suggests the presence of a very sensitive, functionally important, PGE-like system which modulates heart rate in this mammalian species.

Animals↗
PubMed1978

The rates of interaction of local anesthetics with sodium channels in nerve.

Voltage clamp experiments were carried out on Rana catesbiana nodes of Ranvier in order to test predictions regarding the relationship between local anesthetic lipid solubility and the rate of development of and recovery from frequency-dependent increments of sodium channel block. Contrary to expectations, the drugs of greater lipid solubility than lidocaine showed slower rates of development of frequency-dependent block and, in addition, induced longer rather than shorter memories for recent frequency-depent increments in channel block. Relaxation time constants for bupivacaine (50 micrometer), etidocaine (15 micrometer) and tetracaine (0.7 micrometer) measured 50, 8 and 8 sec, respectively, compared to shorter time constant of 2 sec for lidocaine (250 micrometer). Rate constants were calculated for binding to channels in both open and closed states. Open channels displayed a 130- to 6000-fold greater affinity for the local anesthetics than did closed channels, verifying an important feature of the "modulated receptor" hypothesis. In addition, binding to closed channels was enhanced by holding the membrane at more depolarizing potentials, which favored the development of inactive channel states. The exceptionally large binding constants of lidocaine for interactions with both closed and open channels cannot be attributed to its lipid solubility characteristics alone.

Anesthetics, Local↗
PubMed1975

Mechanism of frequency-dependent inhibition of sodium currents in frog myelinated nerve by the lidocaine derivative GEA.

A new lidocaine derivative (Astra, GEA 968) depresses excitability of myelinated frog nerve in a manner which depends upon the rate of use of the nerve. This phenomenon has been shown, under voltage clamp conditions, to involve "frequency-" or "use-dependent" inhibition of the transient inward sodium currents at the node of Ranvier. With 0.6 mM GEA 968 in the solution bathing the node, the inward sodium currents produced by 5-msec depolarizing pulses to -20 mV are reduced to 40% of control values if the node is rested for a few hundred seconds prior to the test pulse. Repetitive opening of the sodium channels by depolarizing pulses enhances this inhibition, for example, currents are eventually reduced to 10 to 20% of control with repetitive depolarization at 2 sec-1. If the preparation is then allowed to rest, this use-dependent increment in inhibition gradually declines with a time constant of about 10 seconds. Repetitive opening of the sodium channels by depolarizing pulses preceded by large hyperpolarizing prepulses reverses the inhibition caused by application of depolarizing pulses alone. It is hypothesized that the GEA 968 molecule binds to open sodium channels and, in doing so, simultaneously blocks the channel and shifts the curve relating sodium inactivation to membrane potential by 20 to 40 mV in the hyperpolarizing direction. Several kinds of evidence supporting this molecular hypothesis are presented. Lidocaine, procaine, procaine amide and a quaternary lidocaine derivative QX-314 also cause use-dependent depression of sodium currents in this preparation. This common mode of action of tertiary and quaternary anesthetics implies that the cationic form of tertiary anesthetics is active.

Animals↗