One might ask why there are so many steps between a challenge, like exercise, and a physiological response like that depicted in Figure 8-2. The answer lies in the ability of biological signal transduction cascades, whose complexity might, at first glance, seem to be almost perverse, to enhance regulatory control. These cascades can amplify, inhibit, and fine-tune responses, integrate signal transduction cascades with one another, and prevent responses from going out of control (“runaway signaling”) by allowing signals to turn themselves off automatically. In fact, the depiction of the signal cascade in Figure 8-2 is oversimplified because most biological signaling pathways are not linear, where each step is coupled to a single downstream reaction, but instead form branches, loop forward and backward, interconnect with other pathways, and generate multiple signals at many steps. These intricacies organize responses that provide options for both amplification and negative feedback, and for the integration of each step in the response with other steps in the cascade and with additional signaling systems. This fine-tuning would not be possible if cell signaling generated only an “all-or-none” response.

An example of a mechanism that regulates signal transduction is the allosteric effect of ATP described in Chapter 7, which accelerates ion fluxes through channels, pumps, and exchangers. This effect is important in energy-starved hearts, where a fall in ATP concentration inhibits several steps in the signaling cascade shown in Figure 8-2; these include the opening of L-type calcium channels in the plasma membrane (step 8) and calcium release channels in the sarcoplasmic reticulum (step 9), and the interactions between actin and myosin (step 12). These and other allosteric effects of decreased ATP concentration help match the rates of ATP production and ATP consumption by inhibiting contraction, the most expensive energy-consuming reaction in the heart. Acidosis, which occurs when glycolysis is accelerated in energy-starved hearts (Chapter 2), also reduces energy consumption by inhibiting many steps in this cascade. A third example, which illustrates how a reaction near the “top” of a signaling pathway can respond to changes further down the cascade, occurs when increased calcium release into the cytosol (step 9) inhibits adenylyl cyclase (step 4); this negative feedback, by slowing the cascade, reduces contractility. Additional mechanisms that avoid calcium overload include the ability of increased cytosolic calcium to accelerate calcium transport out of the cytosol by the plasma membrane and sarcoplasmic reticulum calcium pumps, and by the sodium/calcium exchanger (Chapter 7).

Biological signal transduction cascades often allow a single stimulus to evoke an integrated, multifaceted response. For example, the elevated level of cAMP (step 5 in Fig. 8-2) increases the rate and extent of relaxation when phosphorylation of phospholamban accelerates calcium uptake into the sarcoplasmic reticulum (Chapter 7). Another example of signal diversification occurs at step 3 in Figure 8-2, where norepinephrine binding to a single β₁-receptor activates both G_αs and G_βγ, each of which can modify a different downstream target (see below).

Control of cell function is enhanced further by a large number of isoforms of many signaling molecules. For example, binding of norepinephrine to β₁– and β₂-receptors in the heart evokes different responses, while in vascular smooth muscle, activation of α₁-receptors by norepinephrine causes vasoconstriction, and β-receptor activation has a relaxing effect. These intricacies also
characterize intracellular signal transduction, as is evident by the ability of protein kinases to activate several pathways by phosphorylating a number of different signaling proteins.

The neurohumoral response evoked by exercise, the briefest of the challenges listed in Table 8-1, is initiated by several mechanisms. Most important is the baroreceptor reflex, which increases sympathetic activity and reduces parasympathetic tone in response to the fall in arterial blood pressure caused by vasodilatation in the exercising muscles. Sympathetic activity is also increased when CO₂ and lactic acid released by the exercising muscles stimulate the chemoreceptors. These
responses are supplemented by nerve impulses that arise in the exercising muscles and the cerebral cortex; the latter explains the onset of sympathetic stimulation before the start of exercise, for example, when a sprinter hears the starter’s gun.

The short-term neurohumoral response evoked by exercise is mediated by an inotropic effect that increases the amount of blood ejected during systole, a lusitropic effect that augments filling during diastole, and a chronotropic effect that accelerates heart rate (Tables 8-1 and 8-2). Together, these responses increase the flow of blood out of the heart (cardiac output). Sympathetic stimulation also causes contraction of vascular smooth muscle. Constriction of small arterioles helps maintain blood pressure by increasing the resistance in vascular beds not dilated by exercise, while venoconstriction increases the return of blood to the heart. Fluid retention, as noted above, becomes important only during prolonged exercise.

Shock, a clinical syndrome whose most obvious abnormalities are low blood pressure and inadequate tissue perfusion, also activates the hemodynamic defense reaction. There are many causes of this syndrome, which usually lasts no more than several hours (Table 8-3). Hypovolemic shock occurs when hemorrhage, diarrhea, or leaky capillaries severely decrease circulating blood volume. In distributive shock, which is caused by excessive vasodilatation, there is enough blood in the vasculature but it is in the wrong place, that is, the veins rather than the arteries. Distributive shock can be caused by vasodilator actions of endotoxins (e.g., toxic shock, gram-negative septicemia), reflex vasodilatation (vasovagal syncope, which is the old fashioned “swoon”), and the von Bezolt–Jarisch reflex which can be activated in patients with inferior or posterior myocardial infarction (Chapter 17). A third type of shock, cardiogenic shock, is seen after myocardial infarction, but in this case the cause is reduced ejection by a severely damaged left ventricle. Other causes of cardiogenic shock include valve rupture, pulmonary embolus, pericardial tamponade, acute myocarditis, and arrhythmias. If the underlying cause is reversible and appropriate therapy started promptly, patients usually
recover. However, if treatment is delayed this syndrome becomes irreversible because, even if blood pressure can be increased, the tissue damage initiated by low blood flow causes the patient to die.

The neurohumoral response in shock differs from that evoked by exercise because the challenge is more intense and, more importantly, lasts longer. The hemodynamic defense reaction, therefore, has time to cause the kidneys to retain salt and water, which facilitates restoration of blood volume. However, when vasoconstriction is prolonged and severe, organ damage by low flow is increased.

Patients can recover from shock if the underlying cause is reversible and appropriate treatment is started promptly. Otherwise, severe tissue damage causes this syndrome to become irreversible. If this occurs these patients are doomed to die even if blood pressure can be briefly restored.

In heart failure, where underfilling of the arterial system usually lasts for a lifetime, most of the functional responses listed in Tables 8-1 and 8-2 become maladaptive. Tachycardia and increased contractility, which initially help maintain cardiac output, also increase cardiac energy utilization which, because most failing hearts are energy-starved, contributes to myocardial cell death. Arteriolar vasoconstriction helps maintain blood pressure when the heart fails, but the increased afterload reduces cardiac output and increases myocardial energy demand. Fluid retention by the kidneys, which is beneficial in patients with shock, represents a serious problem in heart failure because the elevated blood volume increases the already elevated pulmonary and systemic venous pressures to levels that cause fluid to be transuded into the lungs, peripheral tissues, and body cavities. Furthermore, the increased preload caused by venoconstriction and fluid retention can do little to increase cardiac output because Starling curves are often flattened in heart failure. For all of these reasons, the neurohumoral response usually becomes deleterious in most patients with heart failure (see Chapter 18).

Cardiac stimulation, vasoconstriction, and salt and water retention, which provide short-term support for the circulation during the brief challenges posed by exercise and shock,
become maladaptive in chronic heart failure. This paradox is vividly summarized by Harris (1983):

Endocrine (hormonal) signaling (Table 8-7) was discovered by Bayliss and Starling (1902), who observed that instilling acid into the duodenum stimulates pancreatic secretion, even after both tissues are denervated. After finding that intravenous injection of a jejunal mucosa extract also
stimulates pancreatic secretion, they proposed that a substance, which they called secretin, travels through the bloodstream from the jejunum, where it is produced, to the pancreas, where it evokes the response. Less than 20 years after this discovery, Loewi described neurotransmitter signaling, where the extracellular messenger is released by the nervous system (see above). More recently, signaling molecules were found to reach their receptors by shorter routes; in paracrine signaling an extracellular messenger released by one cell diffuses to a receptor on a nearby cell, while in autocrine signaling a cell modifies its own function by releasing a ligand that binds to a receptor on its surface. Cytoskeletal signaling allows mechanical interactions within and between neighboring cells and the surrounding extracellular matrix to alter cell structure and function. This type of signaling is initiated when cell deformation modifies signaling proteins—many of which are homologous to the ligands, receptors, enzymes, and other proteins that participate in cell signaling—that interact with the cytoskeleton (see Chapter 5).

Most extracellular messengers contain hydrophobic moieties, so that these molecules have high partition coefficients, a variable that quantifies the distribution of a molecule between the lipid bilayer and the surrounding aqueous medium. Molecules with high partition coefficients tend to accumulate in membranes; for example, when a cell in an aqueous medium encounters 10,001 molecules with a partition coefficient of 10,000, 10,000 of the molecules will enter the bilayer, leaving only 1 in the aqueous medium. Because of their hydrophobicity, extracellular messengers and drugs often have partition coefficients in the thousands. The anti-arrhythmic drug amiodarone has a partition coefficient >1,000,000 (Herbette et al., 1988) that explains its remarkably long biological half-life, which is many months. Their high hydrophobicity allows these molecules to gain access to plasma membrane receptors by first dissolving in the lipid bilayer and then diffusing to their receptors (Herbette et al., 1991). Utilization of this lipid pathway, which allows ligands to gain rapid access to their membrane receptors, also explains why the ligand- and drug-binding sites of many intrinsic membrane proteins include hydrophobic regions of the membrane-spanning α-helices within the bilayer.

The interactions between ligands and their receptors can be characterized in terms of the number of receptors that can bind to the ligand and the affinity with which the ligand attaches to the receptor. Receptor number is often quantified as B_max, the maximal amount of ligand that binds specifically to the receptor. A commonly used index of the affinity of a receptor for its ligand is the dissociation constant (k_d), which is the ligand concentration at which 50% of receptors are bound to the ligand; this means that the greater the affinity of the receptor for the ligand, the lower will be the k_d. The constant k_d also describes the ratio between the rate of ligand dissociation from its receptor (the “off-rate”) and the rate of binding of the ligand (the “on-rate”). Affinity can also be expressed as an association (binding) constant (k_b), which is the reciprocal of k_d, the dissociation constant. Ligands have a wide range of receptor-binding affinities; some peptides bind to their receptors at concentrations below 10¹² M, while most neurohumoral transmitters and drugs occupy their receptors at concentrations between 10⁸ and 10⁶ M. A few compounds, like ethanol, interact with membrane proteins at much higher concentrations (Table 8-8).

Binding affinity is an important determinant of the specificity of the biological response to a ligand. A ligand that binds to its receptor with high affinity usually evokes a specific response
that is accompanied by few side effects; this is because low concentrations of the ligand can be recognized by the receptor, which minimizes interactions of the ligand with other cell components. Ligands that bind to their receptors with low affinity, and so evoke responses only at high concentrations, often have additional “nonspecific” actions that are usually undesirable. The toxic effects of most clinically useful drugs are seen at concentrations higher than those that produce the desired therapeutic effects; exceptions include allergic or sensitivity reactions, which are generally much less dependent on the concentration of the ligand. Specificity can be described as the ratio of the ligand concentrations that cause toxic and therapeutic effects; a higher “toxic/therapeutic ratio” means that a drug is less likely to cause unwanted side effects when administered at doses that yield desirable therapeutic effects.

Characterization of specific receptors made it possible to identify drugs that could inhibit their ability to interact with their ligands. Such inhibitors, often called blockers or antagonists, usually have structures similar to those of the physiological extracellular messengers. The inhibitory effects of many of these drugs exhibit competitive kinetics and can be reversed by high concentrations of the physiological ligand. A few drugs inactivate a signal transduction system completely and permanently; aspirin, for example, acetylates and irreversibly inactivates cyclooxygenase, an enzyme that releases a thrombogenic prostaglandin (see below).

The difference between most receptor blockers (antagonists) and the physiological ligands (agonists) is not where these molecules bind, but what happens after binding has occurred. Unlike an agonist, which activates the subsequent steps in a signal transduction cascade (e.g., after step 1 in Fig. 8-2), antagonists occupy the receptor but do not generate an intracellular signal. The clinical value of an antagonist (e.g., a β-adrenergic blocker) therefore reflects the fact that the receptor-bound antagonist inhibits the ability of the receptor to bind to, and thus become activated by, the physiological agonist (e.g., norepinephrine).

Not all molecules that interact with receptors can be classified simply as agonists and antagonists. Some drugs, called partial agonists, bind to a receptor where they cause a weak activation of its signal transduction cascade, while at the same time blocking the ability of the receptor to interact with the more potent physiological agonists. In the case of the β-adrenergic blockers, the weak stimulatory effect of a partial agonist is referred to as intrinsic sympathomimetic activity (ISA). By blocking the more potent effects of norepinephrine, partial β-adrenergic agonists inhibit the response to surges of sympathetic activity while providing a low level of adrenergic stimulation in the basal state. Unfortunately, many partial agonists have turned out to be less advantageous clinically than this description might suggest.

Cardiovascular function is regulated when a variety of ligands bind to their receptors (Table 8-6). Most functional signals are mediated by G protein-coupled receptors (GPCRs), which are named for the heterotrimeric guanosine triphosphate (GTP)-binding proteins that mediate
their cellular actions (see below). Enzyme-linked receptors, most of which contain a kinase or other enzyme that is activated when the receptor binds to its ligand, were initially thought to mediate only proliferative responses (see Chapter 9); however, these receptors are now known to participate in functional signaling as well. Cytokine receptors resemble enzyme-linked receptors except that instead of having intrinsic enzyme activity, ligand-bound cytokine receptors modify the catalytic activity of other enzymes. Ion channel-linked receptors contain channels that are opened when the receptor binds its ligand. Nuclear receptors bind to hormones, like aldosterone and thyroxin, whose hydrophobic structure allows them to cross the plasma and nuclear membranes; the resulting ligand-receptor complexes regulate gene expression by interactions with DNA in the nucleus. Cellular responses to nitric oxide are not mediated by a receptor; instead, this signaling molecule binds directly to guanylyl cyclase, its target within cells.