Effects of guanyl nucleotides on the activation of cardiac muscarinic receptors

Effects of guanyl nucleotides on the affinity of cholinergic agonists at the [3H]quinuclidinylbenzilate binding site have been compared with their effects on the intact left atrium. The binding of carbachol was described by a two-site model; guanyl nucleotides had little effect on the affinity at either site but increased substantially the proportion of low affinity sites. The negative inotropic effect of acetylcholine (ACh) was unaffected by exogenous guanyl nucleotides or variation in external Mg++ or Ca++. ACh (2 microM) shortened the action potential, reduced spike amplitude and usually increased the maximum diastolic potential. GTP and guanyl-5'-yl-imidodiphosphate in the presence of ACh increased action potential amplitude and duration and increased further the diastolic potential. Low external Mg++ had similar effects to guanyl-5'-yl-imidodiphosphate but their effects were not additive. It is suggested that these compounds delay the activation of a transient outward K+ current by ACh.

Production of pyrimidine dimers in DNA of human skin exposed in situ to UVA radiation

Cyclobutyl pyrimidine dimers, measured as sites recognized by the dimer-specific ultraviolet (UV) endonuclease from Micrococcus luteus, were produced in DNA of human skin exposed in situ to UVA (320-400 nm) radiation. The dimer yields produced by a broadband UVA source, by broadband UVA filtered to remove all light of wavelength less than 340 nm, and by narrow band radiation centered at 365 nm were similar, indicating that UVA radiation, and not stray shorter wavelength radiation, was responsible for dimer production. The identity of the UVA-induced DNA lesions was confirmed as pyrimidine dimers by photoreactivation of approximately 100% of the endonuclease-sensitive sites in vitro with the 40,000 dalton Escherichia coli photoreactivating enzyme.

Quantitation of radiation-, chemical-, or enzyme-induced single strand breaks in nonradioactive DNA by alkaline gel electrophoresis: application to pyrimidine dimers

We have developed an alkaline agarose gel method for quantitating single strand breaks in nanogram quantities of nonradioactive DNA. After electrophoresis together with molecular length standards, the DNA is neutralized, stained with ethidium bromide, photographed, and the density profiles recorded with a computer controlled scanner. The median lengths, number average molecular lengths, and length average molecular lengths of the DNAs can be computed by using the mobilities of the molecular length standards. The frequency of single strand breaks can then be determined by comparison of the corresponding average molecular lengths of DNAs treated and not treated with single strand break-inducing agents (radiation, chemicals, or lesion-specific endonuclease). Single strand break yields (induced at pyrimidine dimer sites in uv-irradiated human fibroblasts DNA by the dimer-specific endonuclease from Micrococcus luteus) from our method agree with values obtained for the same DNAs from alkaline sucrose gradient analysis. The method has been used to determine pyrimidine dimer yields in DNA from biopsies of human skin irradiated in situ. It will be especially useful in determining the frequency of single strand breaks (or lesions convertible to single strand breaks by specific cleaving reagents or enzymes) in small quantities of DNA from cells or tissues not amenable to radioactive labeling.

Modification of the effects of muscarinic agonists by reversible and irreversible anticholinesterase compounds in the guinea pig atrium

The cholinergic agonists acetylcholine (ACh), carbamylcholine and methacholine were found to be equieffective in reducing the force of left atrial contraction, but to differ in their ability to shorten the action potential duration. The irreversible cholinesterase inhibitor soman had no effect on the actions of the non-hydrolyzable agonist carbamylcholine, but potentiated the actions of ACh. The reversible inhibitor edrophonium both potentiated and antagonized the effects of ACh. It antagonized the effects of carbamylcholine and after atrial cholinesterase was inhibited with soman it also antagonized the effects of ACh. Its anticholinesterase action and inhibitory action at the muscarinic receptor were confirmed in separate studies. Edrophonium is approximately 12 times more potent as an anticholinesterase than it is in blocking the muscarinic receptor. However, some actions of edrophonium cannot be explained in the context of its anticholinergic and antiesterase actions. Thus it increases the force of atrial contraction and antagonizes the negative inotropy due to soman. An inhibitory effect on an outward K+ current may be involved. The difference in the ability of the three cholinergic agonists to shorten the action potential may also be related to differences in efficacy at this K+ channel.

Methylation of critical carboxyl groups in the vicinity of the sodium channel of guinea-pig atrium

Methylation of a critical carboxyl group in guinea-pig left atrium with trimethoxonium ion leads to loss of excitability. The critical group(s) could be protected with a number of cationic drugs, so that on washout of the protecting drug and reaction products full excitability returned. Tetrodotoxin, edrophonium, cholinergic agonists and amantadine protected the preparation. During the recovery period readmission of these drugs led to the same pharmacological response as during the control period, suggesting protection of specific site(s). This thesis was confirmed in crossover experiments, in which the atrium was exposed to the methylating agent in the presence of one protecting drug, then exposed to another during recovery. The expected pharmacological responses were obtained. Assay of muscarinic receptors with [3H]-quinuclidinyl benzilate after methylation without protection or in the presence of tetrodotoxin or acetylcholine suggested that these receptors are not involved in the maintenance of excitability. It is postulated that the Na+ channel and the K+ channel are located in the same macromolecular membrane complex, and that the K+ channel has a cholinoreceptive sub-site.

Higher pyrimidine dimer yields in skin of normal humans with higher UVB sensitivity

We have measured UVB (280-320 nm)-induced DNA damage in skin of individuals with different sensitivities to UVB irradiation as measured by minimal erythema dose (MED). The DNA damage was susceptible to cleavage by Micrococcus luteus UV endonuclease, which recognizes pyrimidine dimers in DNA. An alkaline agarose gel electrophoresis method was used to quantitate the number of M. luteus UV endonuclease-sensitive sites in nonradioactive DNA from skin biopsies of 7 individuals irradiated with UVB (0-180 mJ X cm-2). The production of sites correlated well with MED (correlation coefficient = 0.78). The slope of the dose response curve for the most UVB-sensitive individual (MED = 24 mJ X cm-2) and for the least UVB-sensitive individual (MED = 146 mJ X cm-2) were 11.5 X 10(-4) and 2.6 X 10(-4) sites per 1000 bases per mJ X cm-2, respectively. The UVB-induced DNA damage was determined to be pyrimidine dimers by its susceptibility to cleavage by M. luteus UV endonuclease and its photoreactivability by Escherichia coli photoreactivating enzyme.

Serum effects on DNA repair in human cells

Freshly isolated human lymphocytes were used to determine how serum supplements affect cellular capacity to repair UV damage. Repair capacity was always found to be greatest in medium supplemented with autologous plasma. Variability in repair capacity among individuals was greater in serum supplemented medium than in unsupplemented medium. Thus, in vitro cellular responses will most accurately represent in vivo responses if autologous serum factors are present in the culture medium. This is of particular importance in studies attempting to correlate DNA repair capacity with age or susceptibility to carcinogenesis.

Interactions of amantadine with the cardiac muscarinic receptor

The antiviral drug amantadine has anticholinergic effects in the guinea-pig atrium at concentrations greater than 1 X 10(-4)M. It is a competitive inhibitor of [3H]-quinuclidinyl benzilate binding to the muscarinic receptor, but antagonizes the negative inotropic effect of acetylcholine in a non-competitive manner. It increases the duration of the atrial action potential and also increases the force of atrial contraction. These effects are evident at approximately 10 times lower concentrations than the antimuscarinic effects. The increase in contractility can be reversed by propranolol (5 X 10(-7)M) but the increase in action potential duration is potentiated by propranolol. Shortening of the action potential duration by acetylcholine was reversed by amantadine, but at approximately ten times lower levels than were needed to reduce the negative inotropic effect. Interactions between beta adrenoceptor binding of [3H]-dihydroalprenolol and amantadine could not be demonstrated. Similarly, binding of [3H]-nitrendipine to the calcium channel is not influenced. It is suggested that amantadine may exert its positive inotropic effect by interaction with the potassium channel, causing a delay in outward current.

Effect of tetrodotoxin on the inward ionic current of the guinea pig atrium

The inward ionic current in the atrium has been measured by electronic differentiation of the voltage, time signal, on the assumption that the single cell penetrated by the micro-electrode shows cable-like properties. The method exploits fully information contained in the upstroke of the action potential, and an internally consistent set of parameters can be derived. Tetrodotoxin was found to reduce the fast inward ionic current, and to shift the voltage of maximum current flow to more negative values. TTX augmented the effects of high stimulation rates but did not alter the refractory period of the atrium. It is suggested that TTX interacts with the Na+ channel so as to reduce activation of the Na+ current and hence unit conductance. The present results are consistent with the apparent voltage-dependence of TTX action, since activation and inactivation of the Na+ current are both voltage-sensitive phenomena.

Inhibition of unscheduled DNA synthesis in human lymphocytes by chemical carcinogens

Freshly isolated human peripheral lymphocytes were treated with an alkylating agent immediately after collection and subsequently treated with UV radiation. This system was used because it represents a method for assaying damage in cells immediately after their removal from the host. The amount of UV-induced repair was measured as unscheduled DNA synthesis (UDS) by incorporation of [3H]deoxythymidine into the cellular DNA. The alkylating agents beta-propiolactone (BPL) and methyl methane-sulfonate (MMS) inhibited UDS at concentrations of 0.08 mM and 0.6 mM, respectively. Lower concentrations had no effect. Lymphocytes allowed to remain in culture medium after treatment with the alkylating agents did not recover the ability to perform UV-induced UDS even when cells were irradiated 48 h after carcinogen treatment. The decrease in UV-induced UDS resulting from alkylating agent treatment could not be attributed to cell death.

Fine-structure studies of experimental skeletal muscle trauma

A study was made of damage to skeletal muscle caused by a high-velocity rifle bullet. Such damage extends peripherally from the permanent wound cavity and is focal in nature. A fine-structure investigation of this region suggests that some components of the muscle are more susceptible to the wounding process than others. The sarcoplasmic reticulum appeared most sensitive and areas as far as 3 cm from the wound cavity frequently showed gross vacuolization. Mitochrondrial damage was seen, but only in areas where there was also damage to myofibrils and the microvasculature. Focal capillary leakage up to 3 cm from the wound cavity was demonstrated in an earlier study by the use of a fluorescein-labelled dextran (Paddle and Freeman, 1979). This finding was confirmed. A possible correlate at the fine structural level was swelling of te capillary endothelial cells, which occurred in the absence of other signs of microvascular damage. Damage to the endothelial junctions was not observed, even in severely damaged tissue. Intravascular colloidal carbon escaped into the extravascular space only when the microvasculature was fractured. The relationship of these findings to macroscopic damage is discussed.

Cholinergic mechanisms in heart: interactions with 4-aminopyridine

Acetylcholine (ACh) hyperpolarized the guinea-pig atrium and increased the maximum rate of rise of the spike and ionic conductance. There was a dose-dependent decrease in action potential duration in most preparations. However, some atrial showed a contractility decrease without a concomitant change in action potential duration. This was not related to acetylcholinesterase activity. Reduction in contractility by ACh resulted from doses 10 times lower than were required for action potential shortening. Small quantitative differences in electrical response were seen between the left and right atria. 4-Aminopyridine lengthened the action potential and increased spike amplitude. These effects were not frequency-dependent but were potentiated by low Ko+. This drug antagonized both the electrical and contractile effects of ACh, suggesting that they are mediated by an increase in KO+ permeability. Modification of excitation-contraction coupling by ACh is discussed and a role for cyclic guanosine 3':5'-monophosphate suggested.

The effects of l-propranolol and practolol on atrial and nodal transmembrane potentials

Effects of l-propranolol, racemic propranolol and practolol have been determined on transmembrane potentials recorded in guinea-pig left atrium and in sinoatrial and atrioventricular nodes of the rabbit heart. In addition to the voltage -time recording of the action potential, its first time derivative was displayed as a function of membrane voltage, forming a phase-plane trajectory. A number of parameters of the action potential were determined from this trajectory. At the concentrations used the beta adrenoceptor blocking agents reduced the maximum rate of rise of the atrial action potential and slowed repolarization. The velocity of the propagated spike was reduced, and the maximum ionic conductance was also reduced. The excitation potential of the propagated spike was unaffected. The membrane effects were markedly dependent on the frequency of stimulation. The beta adrenoceptor blocking agents were without effect on either the sinoatrial or atrioventricular nodes. Since the spontaneously active right atrium beats more slowly than the heart of the intact rabbit, the atrioventricular node was electrically stimulated at 5 to 6 Hz. Neither drug affected the ability of the node to follow these stimulation frequencies. The importance of these effects in the control of cardiac arrhythmias is discussed.