StAR protein, which is rapidly activated posttranslationally and transcriptionally by pituitary tropic hormones, is short-lived (Fig. 43.7). When there are inactivating mutations in StAR protein, the cells in the zona fasciculata become excessively laden with lipid, resulting in the accumulation of cholesterol because it cannot be accessed by CYP11A1 within the mitochondria to produce cortisol. This condition is known as "lipoid." In these individuals, the synthesis of gonadal steroid hormones is also impaired. It is important to note that the placenta does not express StAR, so normal placental steroid production is observed in utero.

Fig. 43.6 illustrates the individual actions of catecholamines contributing to the integrated sympathoadrenal response to exercise. These actions include bronchiole dilation, exchange of oxygen and carbon dioxide, increased cardiac output, and venous return. Additionally, in the GI and urinary tracts, there is increased motility and energy usage, along with changes in blood glucose, blood ketones, blood free fatty acids, blood lactate, and blood glycerol levels. Adipose tissue experiences lipolysis, while the liver undergoes glycogenolysis, gluconeogenesis, and ketogenesis. These processes are reinforced by hormones. In skeletal muscle, there is glycogenolysis and increased glucose uptake, while in pancreatic alpha cells, glucagon secretion is stimulated. The blood glucagon/insulin ratio is also affected, ensuring nutrient supply to the muscles and adequate oxygen and glucose supply to the brain. Blood flow to the GI tract decreases, while blood flow to skeletal muscle increases. Splanchnic arterioles undergo vasoconstriction, and skeletal muscle arterioles experience vasodilation. Veins and lymphatics also undergo vasoconstriction, while the heart exhibits positive inotropic, positive chronotropic, and positive lusitropic effects (Fig. 43.7).

Fig. 43.7 depicts the events involved in the first two reactions of the steroidogenic pathway: the conversion of cholesterol to pregnenolone and the conversion of pregnenolone (P5) to progesterone (P4) in zona fasciculata cells. Various enzymes and molecules participate in these reactions, including acylCoA:cholesterol acyltransferase (ACAT), 3β hydroxysteroid dehydrogenase (3β HSD), cholesterol esters (CE), P450 side-chain cleavage enzyme (also known as CYP11A1), free cholesterol (FC), hormone-sensitive lipase (HSL), lysosomal acid hydrolase (LAL), low-density lipoprotein (LDL), low-density lipoprotein receptor (LDLR), pregnenolone (P5), and steroidogenic acute regulatory protein (StAR).

Fig. 43.8 provides a summary of the steroidogenic pathways in each of the three zones of the adrenal cortex. The enzymatic reactions are color-coded across the zones, and the smooth endoplasmic reticulum (sER) is involved in these processes.

(Modified from White BA, Porterfield SP. Endocrine and Reproductive Physiology. 4th ed. Philadelphia: Mosby; 2013.)
Fig. 43.7). StAR protein is short-lived and rapidly activated posttranslationally (phosphorylation) and transcriptionally by pituitary tropic hormones. In patients with inactivating mutations in StAR protein, cells of the zona fasciculata become excessively laden with lipid (“lipoid”) because cholesterol cannot be accessed by CYP11A1 within the mitochondria and used for cortisol •Fig. 43.6 Someoftheindividualactionsofcatecholaminesthatcontributetotheintegratedsympa-thoadrenalresponsetoexercise.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine and Reproductive Physiology. 4thed.Philadelphia:Mosby;2013.)Bronchiole (˜2) Dilation ° Exchange of O2 and CO2 ° Cardiac output ° Venous return GI and urinary tracts (˜2) ˝ Motility ˝ Energy usage ° Blood glucose ° Blood ketones ° Blood FFAs ° Blood lactate ° Blood glycerol Adipose (˜2, ˜3) ° Lipolysis ˝ Glucose uptake Liver (˜2) ° Glycogenolysis ° Gluconeogenesis ° Ketogenesis Hormonal reinforcement Skeletal muscle (˜2) ° Glycogenolysis ˝ Glucose uptake ˜ Cells (˙2) ˝ Insulin secretion ˙ Cells (˜2) ° Glucagon secretion ° Blood glucagon/insulin ratio ° Nutrient supply to muscle and adequate supply of oxygen and glucose to brain ˝ Blood flow to GI tract ° Blood flow to skeletal muscle Splanchnic arterioles (˙) Vasoconstriction Skeletal muscle arterioles (˜2) Vasodilation Veins and lymphatics (˙) Vasoconstriction Heart (˜1) (+)ˆve Inotropic (+)ˆve Chronotropic (+)ˆve Lusitropic •Fig. 43.7 Eventsinvolvedinthefirsttworeactionsinthesteroidogenicpathway:conversionofcholesteroltopregnenolone;conversionofpregnenolone(P5)toprogesterone(P4)inzonafasciculatacells.ACAT,acylCoA:cholesterolacyltransferase;3β HSD,3β hydroxysteroiddehydrogenase;CE,cholesterolesters;CYP11A1alsocalledP450 side-chain cleavage enzyme; FC,freecholesterol;HSL,hormone-sensitivelipase;LAL,lysosomalacidhydrolase;LDL,low-densitylipoprotein;LDLR,low-densitylipoproteinreceptor;P5,pregnenolone;StAR,steroidogenicacuteregulatoryprotein.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine and Reproductive Physiology. 4thed.Philadelphia:Mosby;2013.) •Fig. 43.8 Summaryofthesteroidogenicpathwaysforeachofthethreezonesoftheadrenalcortex.Theenzymaticreactionsarecolorcodedacrosszones.sER,smoothendoplasmicreticulum.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine and Reproductive Physiology. 4thed.Philadelphia:Mosby;2013.) synthesis. Moreover, these individuals cannot synthesize gonadal steroid hormones. The placenta does not express StAR, so these individuals have normal placental steroid production in utero.