CVS, Other Drug Models & Biostatistics
This comprehensive final unit covers preclinical screening models for the Cardiovascular System (antihypertensives, diuretics, antiarrhythmics, anticoagulants), followed by models for other critically important drug categories (antiulcer, antidiabetic, anticancer, antiasthmatic). It concludes with the essential Research Methodology and Biostatistics tools needed to design experiments, test hypotheses, and statistically analyze preclinical data using Student’s t-test and One-way ANOVA.
Syllabus & Topics
- 1CVS Screening – Antihypertensives & Diuretics: Antihypertensive Models: DOCA-Salt Hypertension (Rat): Chronic administration of Deoxycorticosterone Acetate (DOCA) + high salt diet + uninephrectomy induces sustained hypertension. The test antihypertensive’s ability to lower BP is evaluated by tail-cuff plethysmography (non-invasive) or carotid cannulation (invasive). Two-Kidney One-Clip (2K1C) Goldblatt Model: A silver clip partially occludes one renal artery, activating the Renin-Angiotensin system and causing renovascular hypertension. SHR (Spontaneously Hypertensive Rats): Genetically hypertensive inbred strain—ideal for chronic antihypertensive drug testing. Diuretic Models: Lipschitz Method: Rats are given a large oral water load (25 mL/kg 0.9% NaCl). Test diuretic is administered. Urine is collected in metabolic cages for 5-24 hours. Volume, Na⁺, K⁺, Cl⁻ electrolyte concentrations are measured. The diuretic index = (urine volume of test group / urine volume of control group) is calculated.
- 2CVS Screening – Antiarrhythmic, Anticoagulant & Anti-aggregatory Models: Antiarrhythmic Models: Aconitine-Induced Arrhythmia (Rat): IV infusion of Aconitine (sodium channel opener) induces a sequence of arrhythmias (PVCs → VT → VF). The dose/time to onset of arrhythmia is measured. Antiarrhythmic drugs delay onset or prevent arrhythmia. BaCl₂-Induced Arrhythmia: IV Barium Chloride induces reversible ventricular arrhythmias by depolarizing cardiac cells. Chloroform-Induced VF (Mouse): A brief inhalation of chloroform triggers ventricular fibrillation. Anticoagulant & Coagulant Models: Bleeding Time (Tail-Tipping Method in mice): The tail tip is cut, and the time until bleeding stops is recorded. Anticoagulants prolong bleeding time. Clotting Time (Capillary Tube Method): Blood is drawn into a glass capillary tube; the time to clot formation is recorded. Prothrombin Time (PT) and Activated Partial Thromboplastin Time (APTT): Measured using plasma and standard reagents. Anti-aggregatory: Platelet Aggregation Studies using a platelet aggregometer (Born’s method) with ADP/Collagen/Arachidonic acid as aggregating agents.
- 3Other Drug Screening Models – Antiulcer & Antidiabetic: Antiulcer Models: Pylorus Ligation Model (Shay Rat Model): The pylorus is surgically ligated under anesthesia. After 4 hours, the animal is sacrificed, and gastric acid volume, pH, and total acidity are measured. Antiulcer drugs reduce acid secretion. Ethanol-Induced Gastric Ulcer: Oral administration of absolute ethanol (1 mL/200g) causes severe hemorrhagic gastric lesions. The ulcer index (number × severity scored 0-3) is calculated. Cytoprotective drugs (like Sucralfate, Misoprostol) reduce the ulcer index. Aspirin/Indomethacin-Induced Ulcer: NSAID-induced gastric damage models COX-inhibition-mediated mucosal injury. Antidiabetic Models: Alloxan-Induced Diabetes (Mouse/Rat): Alloxan (IP/IV) selectively destroys pancreatic β-cells through reactive oxygen species generation, inducing permanent Type 1-like diabetes (blood glucose >250 mg/dL). Streptozotocin (STZ)-Induced Diabetes: STZ is more selective and dose-dependent. Low dose: partial β-cell destruction → Type 2-like model. High dose: Complete destruction → Type 1 model. Fasting blood glucose is measured using a glucometer at day 0, 7, 14, 21, 28.
- 4Other Drug Screening Models – Anticancer & Antiasthmatic: Anticancer Models: In-vitro: MTT Assay—Cancer cell lines (MCF-7 breast cancer, HeLa cervical) are treated with the test drug. MTT dye is added; living cells convert MTT to purple Formazan crystals. Absorbance is proportional to viable cell count. IC50 (concentration killing 50% cells) is calculated. In-vivo: Ehrlich Ascites Tumor (EAT) Model—EAT cells are injected IP into mice. Test anticancer drug is administered, and tumor progression, body weight, and survival time are measured. Dalton’s Lymphoma Ascites (DLA) model is similar. Antiasthmatic Models: Histamine/Acetylcholine-Induced Bronchospasm (Guinea Pig): Guinea pigs are exposed to nebulized Histamine (0.2%) or Acetylcholine in an aerosol chamber. The Pre-Convulsion Time (PCT—time to onset of dyspnea/convulsions) is measured. Antiasthmatic drugs (Salbutamol, Chlorpheniramine) significantly increase PCT. Sensitized Guinea Pig Model: Animals pre-sensitized with Ovalbumin; asthmatic response triggered by aerosolized Ovalbumin challenge.
- 5Research Methodology: Selection of Research Topic: Identify gaps in existing literature through systematic literature review. Formulate a clear, testable Research Question. Review of Literature: PubMed, Scopus, Google Scholar searches using MeSH terms. Research Hypothesis: Null Hypothesis (H₀): ‘The test drug has NO significant effect.’ Alternative Hypothesis (H₁): ‘The test drug HAS a significant effect.’ Study Design: Randomization of animals into groups, blinding (single/double-blind), sample size calculation (power analysis), choice of dose levels, and duration of study.
- 6Biostatistics – Data Analysis: Student’s t-test: Compares means of TWO groups. Unpaired t-test: Compares two independent groups (e.g., Control vs. Test drug). Paired t-test: Compares the same group before and after treatment. Calculates t-value → compared against critical t-value at chosen significance level (usually p<0.05). One-Way ANOVA (Analysis of Variance): Compares means of THREE or more groups simultaneously. F-ratio = (Between-group variance) / (Within-group variance). If F-calculated > F-critical at p<0.05, at least one group is significantly different. Post-hoc tests (Tukey’s, Bonferroni, Dunnett’s) identify WHICH specific groups differ. Graphical Representation: Bar graphs with error bars (Mean ± SEM), dose-response curves, box plots. Statistical software: GraphPad Prism, SPSS, R.
Learning Objectives
Exam Prep Questions
Q1. Why do we use Alloxan or Streptozotocin to induce diabetes in rats rather than simply starving them?
Starvation reduces blood glucose levels but does not affect insulin production. Diabetes mellitus is primarily caused by the loss or dysfunction of insulin-producing β-cells. Alloxan and streptozotocin selectively target these β-cells because they are taken up via the GLUT2 transporter. Once inside, they generate reactive oxygen species that damage cellular DNA, leading to β-cell destruction. This creates a condition that closely mimics insulin deficiency seen in human diabetes.
Q2. Why use ANOVA instead of running multiple t-tests when comparing 4 groups?
Using multiple t-tests for several group comparisons increases the risk of Type I error (false positives). For example, comparing 4 groups requires 6 pairwise tests, significantly raising the overall probability of error. ANOVA (Analysis of Variance) solves this by performing a single test that maintains the overall error rate at 5%. If ANOVA shows a significant result, post-hoc tests (like Tukey’s test) are then used to identify specific group differences.
Q3. What exactly does the MTT assay measure?
The MTT assay measures cell viability and metabolic activity. MTT is a yellow dye that is reduced by mitochondrial enzymes in living cells to form purple formazan crystals. Dead cells cannot perform this conversion. After dissolving the crystals, the absorbance is measured spectrophotometrically. Higher absorbance indicates more viable cells, while lower absorbance after drug treatment indicates increased cytotoxicity.