The adrenergic innervation of pulmonary vasculature in the normal and pulmonary hypertensive rat.

It would appear that susceptibility to chronic proliferative pulmonary hypertension in response to chronic alveolar hypoxia is most severe in species in which adrenergic innervation of pulmonary arteries is reduced or lacking. Intrapulmonary arteries of the rat have been reported to lack adrenergic innervation by some workers but not others. Since the rat develops severe proliferative pulmonary hypertension in response to prolonged alveolar hypoxia, the different divisions of the lung vasculature of Sprague-Dawley rats were thoroughly examined to determine the presence or absence of an adrenergic innervation. The degree of innervation in normal rats was compared with that of rats developing pulmonary hypertension. Both in normal and experimental pulmonary hypertensive rats the pulmonary arteries, all their branches and the small pulmonary veins with a smooth muscle media were found to be devoid of adrenergic innervation. In contrast, the cardiac-like muscle in the media of large pulmonary veins, the bronchial arteries and the vasa vasorum of larger vessels were richly innervated by adrenergic nerves. Thus the increase in medial smooth muscle which occurs in pulmonary arteries during chronic alveolar hypoxia is independent of a pre-existing adrenergic innervation or of such an innervation newly derived from that of adjacent vessels or structures. This is in contrast to systemic vessels where it has been suggested that increased adrenergic activity and density of innervation may augment hypertrophy of the media in hypertensive animals. Adrenergic nerves are suggested to have a protective action on pulmonary vessels.

Membrane properties of smooth muscle cells in pulmonary arteries of the rat.

Electrical properties of the membrane of smooth muscle cells in the rat main pulmonary artery (MPA) and a small pulmonary artery (SPA) were compared. MPA and SPA differed in several important respects, suggesting characteristic quantitative and qualitative differences in membrane properties. 1) Resting membrane potentials were similar in both (MPA 52.2 +/- 1.3 mV; SPA 51.5 +/- 1.7 mV). The cells displayed no spontaneous electrical activity. The muscle layers in both MPA and SPA showed cablelike properties; a graded local response to outward current pulses was observed, but no action potentials were evoked. 2) Tetraethylammonium chloride (TEA, 1-5 mM) depolarized, increased membrane resistance, and suppressed rectification in MPA. TEA strongly depolarized SPA and contraction ensued. 3) The maximum membrane depolarization produced by a 10-fold increase in extracellular [K+] was 48 mV in MPA and 47 mV in SPA. In K+-free solution gradual depolarization was observed in SPA, but the membrane potential in MPA was not modified. Restoration of K+-containing solution produced equivalent hyperpolarization in both tissues, indicating a similar degree of stimulation of electrogenic Na+-K+ pumping. 4) A Na+-deficient solution did not affect the membrane potential in MPA but depolarized SPA.

Membrane properties of smooth muscle cells in pulmonary hypertensive rats.

The membrane properties of smooth muscle cells in rat main pulmonary artery (MPA) and small pulmonary artery (SPA) were investigated during chronic normobaric hypoxia and after monocrotaline injection. As chronic pulmonary hypertension developed, pronounced differences between MPA and SPA were observed. These findings may shed light on mechanisms of smooth muscle hypertrophy. 1) The resting membrane potential of smooth muscle in MPA became less negative than the normal (depolarized), whereas the resting membrane potential of smooth muscle in SPA became more negative (hyperpolarized). 2) In MPA, both the length and time constants diminished. 3) In MPA, the maximum membrane depolarization produced by a 10-fold increase in extracellular [K+] decreased. 4) In SPA, the depolarization observed in K+-free solution was more rapid and greater in amplitude, and the transient hyperpolarization following restoration of K+-containing solution increased. 5) In SPA, initial and sustained depolarization evoked by Na+-deficient solutions were increased. 6) Depolarization in MPA was due to increased membrane permeability, perhaps to Cl-, whereas hyperpolarization in SPA could be attributed to increased activity of an electrogenic Na+-K+ pump.

Lanthanum block of contraction and of relaxation in response to serotonin and dopamine in molluscan catch muscle.

Contractile and relaxing responses of the anterior byssus retractor muscle of Mytilus edulis L. were observed after exposure to La+++. After 25 minutes in 5 mM La+++, contraction in response to acetylcholine, to KCl and to stimulation of intramuscular nerves is blocked, whereas contraction in response to direct current pulses is partially blocked and caffeine contraction is unaffected. Exposure to 5 mM La+++ for 3 hours does not block relaxation of catch tension in response to serotonin and dopamine but exposure to 5 mM La+++ in the presence of a depolarizing agent (acetylcholine or KCl) for a brief period (4 minutes) abolishes the relaxing response. The authors concluded that activation of contraction in the anterior byssus retractor muscle can be effected by Ca++ influx from the bathing medium and/or by release of Ca++ from interal sites, depending on the nature of the stimulation. The authors also concluded that the relaxing action of serotonin and dopamine depends neither on Ca++ influx nor on passive efflux of Ca++, but rather on Ca++ combining at an internal site or at sites on the cell membrane which are exposed when the muscle is depolarized.

Serotonin and dopamine as neurotransmitters in mytilus: block of serotonin receptors by an organic mercurial.

The effects of mersalyl, bromo-LSD (BOL) and methysergide (UML) on the relaxation of catch by certain indole and catechol derivatives were studied in the anterior byssus retractor muscle of Mytilus. Mersalyl antagonized relaxation in response to serotonin whereas BOL and UML were less effective. Two other indole derivatives, ergotamine and gramine, were also blocked by mersalyl; BOL and UML antagonized relaxation in response to dopamine more effectively than did mersalyl. Two other catechols, epinephrine and norepinephrine, were also blocked more effectively by BOL and UML than by mersaly. Relaxation in response to neural stimulation was blocked more effectively by mersalyl than by BOL. The blocking action of mersalyl on neural relaxation reversed very poorly after washing the drug, but complete reversal was induced by brief exposure to dithiothreitol. It is concluded that the evidence supports an hypothesis that the transmitter released by relaxing nerves is serotonin. It is suggested that mersalyl blocks serotonin by combining with a sulfhydryl group at or near the site on the receptor to which the indole nitrogen attaches.

Aspects of smooth muscle function in molluscan catch muscle.

1) Catch in Mytilus ABRM may be a specialization of a mechanism common to all muscles that gives rise to stretch resistance in the resting state. Catch appears to be due to actin myosin interaction. Since this interaction is regulated by nerves, it provides a convenient model for studying resting stretch resistance. 2) Studies of the structure of Mytilus ABRM revela two types of intercellular connections: a) direct connections between muscle fibers [these nexal (gap) junctions interconnect the muscle cells electrically]; b) muscle fiber-collagen-muscle fiber connections [these provide mechanical connections between muscle cells via collagen fibers]. The structure of Mytilus ABRM supports speculation that smooth muscle filaments are organized into contractile units. 3) A rise in cAMP levels occurs in response to the relaxing transmitter, serotonin. It is not certain whether the cAMP system directly controls the ability of the contractile proteins to interact or whether it regulates intracellular levels of Ca2+. 4) Calcium ions in activation are derived from two sources: an internal source, probably the sarcoplasmic reticulum, and an external source, across the muscle membrane. 5) The nature of catch remains in question, although most evidence favors the linkage hypothesis.

The structure of Mytilus smooth muscle and the electrical constants of the resting muscle.

The individual muscle fibers of the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. are uninucleate, 1.2-1.8 mm in length, 5 microm in diameter, and organized into bundles 100-200 microm in diameter, surrounded by connective tissue. Some bundles run the length of the whole muscle. Adjacent muscle cell membranes are interconnected by nexuses at frequent intervals. Specialized attachments exist between muscle fibers and connective tissue. Electrical constants of the resting muscle membrane were measured with intracellular recording electrodes and both extracellular and intracellular current-passing electrodes. With an intracellular current-passing electrode, the time constant tau, was 4.3 +/- 1.5 ms. With current delivered via an extracellular electrode tau was 68.3 +/- 15 ms. The space constant, lambda, was 1.8 mm +/- 0.4. The membrane input resistance, R(eff), ranged from 23 to 51 MOmega. The observations that values of tau depend on the method of passing current, and that the value of lambda is large relative to fiber length and diameter are considered evidence that the individual muscle fibers are electrically interconnected within bundles in a three-dimensional network. Estimations are made of the membrane resistance, R(m), to compare the values to fast and slow striated muscle fibers and mammalian smooth muscles. The implications of this study in reinterpreting previous mechanical and electrical studies are discussed.