Bioassay and Clinical Endocrine & Cardiac Concepts

Posted on Jan 13, 2026 in Medicine

Bioassay

Bioassay is defined as the estimation of the potency of an active principle in a unit quantity of a preparation.

• Detection and measurement of the concentration of the substance in a preparation using biological methods.

Importance of Bioassay

Bioassays, as compared to other methods of assay (e.g., chemical or physical assay), are very important because they are the only method of assay if:

  1. Active principle of a drug is unknown or cannot be isolated (e.g., insulin, posterior pituitary extract).
  2. Chemical method is either not available or, if available, is too complex and insensitive or requires higher dose (e.g., insulin, acetylcholine).
  3. Chemical composition is not known (e.g., long-acting thyroid stimulants).
  4. Chemical composition of drugs differs but they have the same pharmacological action and vice versa (e.g., cardiac glycosides, catecholamines).

Principle of Bioassay

The basic principle of bioassay is to compare the test substance with the International Standard preparation of the same substance and to find out how much test substance is required to produce the same biological effect as produced by the standard.

Quantal Assay

Quantal response: the response is in the form of “all or none,” i.e., either no response or maximum response.

Drugs producing quantal effects can be bioassayed by the end-point method.

Graded Assay

Graded response: response is proportional to the dose and may lie between no response and the maximum response.

Types:

  • Bracketing / direct matching
  • Interpolation

Multiple point assays: Three point assay, Four point assay, Six point assay • Cumulative dose response

Diabetes Mellitus

“Diabetes is a group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both.”

The chronic hyperglycemia of diabetes is associated with long-term damage, dysfunction, and failure of different organs, especially the eyes, kidneys, nerves, heart, and blood vessels.

Criteria for Diagnosing Diabetes

According to WHO, diabetes is diagnosed if:

  • Fasting glucose ≥ 7 mmol/L (≥ 126 mg/dL)
  • Random glucose ≥ 11.1 mmol/L (≥ 200 mg/dL)

Clinical Classification of Diabetes

There are four clinical classifications of diabetes:

  • Type 1 diabetes (insulin-dependent diabetes mellitus)
  • Type 2 diabetes (non-insulin-dependent diabetes mellitus)
  • Gestational diabetes
  • Diabetes due to other causes (genetic defects or medications, etc.)

Type 1 Diabetes Mellitus

• Also known as juvenile-onset diabetes. Complete or near-total insulin deficiency. Occurs < 30 years of age. Underweight patient. Shows classic symptoms of insulin deficiency (polydipsia, polyphagia, polyuria, and weight loss). Require exogenous (injected) insulin to control hyperglycemia and maintain blood glucose concentrations as close to normal as possible.

Type 2 Diabetes Mellitus

• Most common.

• Typically > 40 years of age. Normal or overweight. Genetic factor; strongly positive family history.

Etiology

1. Insulin resistance.

  • Initially normal or increased insulin secretion, but later decreases.
  • β-cells are usually normal but there is less active insulin production.
  • Less cellular response to insulin due to deficiency of GLUT4 transporter in insulin-sensitive cells.
  • Presence of antibodies against insulin (in some cases).

Thyroid Gland

The thyroid gland and parathyroid glands are a group of endocrine glands.

Thyroid gland secretes three hormones:

  • Triiodothyronine (T3)
  • Thyroxine (T4)
  • Calcitonin

T3 and T4 are produced by thyroid follicles and have similar biological activity.

Calcitonin, produced by interfollicular ‘C’ cells, is chemically and biologically entirely different.

Calcitonin is considered along with parathormone (PTH), with which it regulates calcium metabolism.

Thyroxine was the first hormone synthesized in the laboratory.

The thyroid gland and parathyroid glands are located in the base of the neck. These glands play a vital role in maintaining the body’s homeostasis by producing hormones that regulate metabolism and free calcium levels.

Synthesis

Steps involved:

IODIDE TRAPPING ^ SYNTHESIS OF THYROGLOBULIN (TGB) ^ OXIDATION OF IODIDE ^ IODINATION OF TYROSINE ^ COUPLING OF T1 & T2 ^ PINOCYTOSIS & DIGESTION OF COLLOIDS ^ SECRETION OF THYROID HORMONES ^ TRANSPORT IN THE BLOOD

Iodide Trapping

Follicular cells trap iodide ions by the Na+-iodide transporter.

Synthesis of Thyroglobulin (TGB)

TGB is a large glycoprotein synthesized in the rough endoplasmic reticulum of follicular cells. It is modified and stored in secretory vesicles by the Golgi apparatus and then released into the lumen of thyroid follicles.

Oxidation of Iodide

For the synthesis of thyroid hormones, the amino acids (i.e., tyrosine) of TGB need to be iodinated.

Negatively charged iodide ions cannot iodinate tyrosine directly. They undergo oxidation by the enzyme peroxidase to form iodine.

Iodination of Tyrosine

If one iodine reacts with tyrosine, monoiodotyrosine (MIT, T1) is formed.

If two iodines react with tyrosine, diiodotyrosine (DIT, T2) is formed.

Colloid: The TGB with attached iodine atoms is stored in the lumen of the thyroid follicle.

Coupling T1 & T2

▸ Two molecules of T2 join to form T4 (tetraiodothyronine).

▸ One molecule of T2 and one molecule of T1 join to form T3 (triiodothyronine).

Pinocytosis and Digestion of Colloids

Droplets of colloid re-enter follicular cells by pinocytosis. They fuse with lysosomes. The digestive enzyme protease in lysosomes breaks down thyroglobulin and releases T3 and T4.

Secretion of Thyroid Hormones

As T3 and T4 are lipid soluble, they diffuse through the plasma membrane and reach the blood circulation.

Transport in the Blood

T3 and T4 are transported in the blood by combining with transport proteins, e.g., thyroxine-binding proteins.

Regulation of Thyroid Hormone Secretion

To maintain normal levels of metabolic activity in the body, precisely the right amount of thyroid hormone must be secreted at all times. To achieve this ideal level of secretion, specific feedback mechanisms operate through the hypothalamus and anterior pituitary gland to control the rate of thyroid secretion.

Hemodynamics and Blood Composition

Hemodynamics is the application of physical and physiological principles of blood flow (circulatory system) in the body.

Blood Composition

– Approx. 45% by volume: solid components → Red blood cells (12 µm × 2 µm), white cells, platelets.

– Approx. 55%: liquid (plasma) → 91.5% of which is water, 7% plasma proteins, 1.5% other solutes.

Blood functions:

  • Transportation of blood gases, nutrients, wastes
  • Homeostasis (regulation) of pH, body temperature, water content
  • Protection

Electro-Cardiac Physiology

Parts of the heart involved in conduction:

Sinus node, atrium, AV node, Bundle of His, left bundle branch, right bundle branch, ventricles.

The main functions of the heart are to pump blood through circuits:

  • Pulmonary circuit: through the lungs to oxygenate the blood and remove carbon dioxide
  • Systemic circuit: to deliver oxygen and nutrients to tissues and remove carbon dioxide

➤ A dual pump.

In order to beat, the heart needs three types of cells:

  • Rhythm generators to produce an electrical signal — SAN / pacemaker
  • Conductors to spread the pacemaker signal
  • Contractile cells (myocardium) to mechanically pump blood

Properties of myocardial cells: automaticity, excitability, conductivity, contractility.

Depolarization

Movement of ions (Na+, K+ and Ca2+) across the membrane causing the inside of the cell to become more positive; an electrical event which is expected to result in a contraction (a mechanical event). A difference between electrical charges must exist for electrical current to be generated.

This electrical activity appears on an ECG as waveforms.

Repolarization

Movement of ions across the cell membrane in which the inside of the cell is restored to its negative charge.

Cardiac Electrophysiology

Cardiac electrophysiology is the study of elucidating, diagnosing, and treating the electrical activities of the heart. Its purposes are:

  • to assess arrhythmias
  • to elucidate symptoms
  • to evaluate abnormal electrocardiograms
  • to assess risk of developing arrhythmias in the future
  • to design treatment

Normal Heart Conduction

An electrical impulse stimulates the heart muscle to contract. Normal electrical conduction starts in the sino-atrial (SA) node sending an impulse through the atria to the atrio-ventricular (AV) node, which is the relay station of the heart. It sends the electrical impulses to the ventricles.

Cardiac Action Potential

The cardiac action potential, the basic unit of electrical activity in the heart, produces cardiac contractions. Cardiac myocytes have a resting membrane potential of approximately -90 mV (polarized). Under the influence of trigger events, potassium, sodium, and calcium ions cross the cell membrane, thereby generating discrete ion currents.

Phases of the Cardiac Action Potential

Divided into five phases:

  • Phase 0: Opening of Na+ channels
  • Phase 1: Opening of K+ channels
  • Phase 2: Opening of Ca2+ channels
  • Phase 3: Opening of K+ channels
  • Phase 4: Return to resting membrane potential

Hypertension

Introduction

Hypertension is an elevation of systolic and diastolic blood pressure above 140/90 mmHg. In hypertension the heart is working harder than normal, putting extra strain on the heart and vessels.

Blood Pressure

Systolic blood pressure: the amount of pressure against the artery walls each time the heart contracts.

Diastolic blood pressure: the amount of pressure inside the artery when the heart is at rest, in between heartbeats.

Blood pressure is controlled by baroreceptor reflexes acting through the autonomic nervous system along with the renin-angiotensin-aldosterone system.

Types of Hypertension

  • Primary (Essential) Hypertension: cause is not known.
  • Secondary Hypertension: causes are known (kidney problems, adrenal gland tumor — aldosteronism, thyroid problems — increased Ca2+).

Risk Factors

Risk increases with age (above 65 years), family history, tobacco smoking, diet (high salt and oil intake and alcohol), and stress.

Antihypertensive Drug Classes and Examples

2. ACE inhibitors: Captopril, Lisinopril, Enalapril, Ramipril.

3. Angiotensin receptor blockers: Losartan, Candesartan, Valsartan.

4. Renin inhibitors: Aliskiren, Remikiren.

5. β-adrenergic blockers:

  • (a) Non-selective: Propranolol, Timolol.
  • (b) Cardioselective: Bisoprolol, Metoprolol, Atenolol.

7. α-β adrenergic blockers: Labetalol, Carvedilol.

8. Calcium channel blockers:

  • (a) L-type: Verapamil, Diltiazem, Efonidipine.
  • (b) Dihydropyridine: Nifedipine, Nicardipine, Lacidipine.

9. Potassium channel openers: Minoxidil, Diazoxide.

10. Vasodilators:

  • (a) Arteriolar dilators: Hydralazine, Minoxidil.
  • (b) Arteriolar-venodilator: Nitroprusside sodium.

Classification of Hypertension in Pregnancy

  • Pre-eclampsia and eclampsia syndrome
  • Chronic hypertension
  • Pre-eclampsia superimposed on chronic hypertension
  • Gestational hypertension
  • Post-partum hypertension

Hypertension Severity in Pregnancy

Mild: Systolic BP > 140 mmHg or diastolic BP > 90 mmHg on 2 occasions at least 4 hours apart while seated at rest, after 20 weeks.

Severe: Systolic BP > 160 mmHg or diastolic BP > 110 mmHg while seated at rest, after 20 weeks.

Hydralazine (Apresoline)

MOA: direct vasodilator.

Advantage: rapid onset, improves placental & renal blood flow, no fetal side effects reported.

Disadvantage: tachycardia, palpitations, headache, flushing.

Acute Management Protocol (Nifedipine and Hydralazine)

Administer 10 mg Nifedipine tablet orally. Monitor BP every 15 minutes. If after 45 minutes severe hypertension persists, give a second dose of 10 mg Nifedipine tablet orally. Monitor BP every 15 minutes until BP stabilizes. If after 45 minutes from the second dose (90 minutes from the first dose) severe hypertension persists:

Dilute 20 mg Hydralazine in 20 mL of water for injection. Administer 5 mg (5 mL) as an IV bolus. Monitor BP every 10 minutes. If after 20 minutes severe hypertension persists, administer a second dose of 5 mg (5 mL) Hydralazine. If after another 20 minutes the same findings persist, administer a third dose of 5 mg (5 mL) Hydralazine. If severe hypertension persists after 3 boluses of IV hydralazine:

Draw 10 mL out of a 500 mL normal saline bag, mix the 10 mL with 80 mg hydralazine powder, and then load it back into the 500 mL bag. Start hydralazine infusion via pump at 30 mL/hr (5 mg/hr). Increase infusion by 10 mL every 30 min to a maximum of 90 mL/hr (15 mg/hr), aiming for systolic 140–160 mmHg and diastolic 90–100 mmHg.

5 or 10 mg IV Hydralazine over 2 minutes.

20 minutes • 10 mg IV Hydralazine over 2 minutes.

20 minutes • 20 mg IV Hydralazine over 2 minutes.

20 minutes • 40 mg IV Hydralazine over 2 minutes.

Labetalol — Mechanism of Action

Blocks β-1, β-2, and α-1 sympathetic receptors. It mainly acts by decreasing peripheral vascular resistance. Cardiac output is not significantly affected. It has no effect on utero-placental blood flow.