Brand Name: Norvasc, Istin

Available in 2.5 mg, 5 mg, and 10 mg tablets

Background

Muscle is ultimately composed of protein filaments. These filaments are arranged in parallel in an overlapping fashion. When muscles contract, the fibers are able to slide across one another, shortening the overall length to up to one-third of the original length.

Skeletal Muscle

Microscopic view of a cross-section of skeletal muscle. Image Courtesy of National Toxicology Program, U.S. Dept of Health and Human Services.

Muscle contraction is made possible by the release of calcium from storage within the muscle cell (in a structure called the sarcoplasmic reticulum). Nerve stimulation causes the release of calcium, which allows muscle contraction to happen. No calcium, no muscle contraction.

There are three types of muscle within the body: skeletal muscle (the muscles under voluntary control that we use to move, as shown above); smooth muscle, the involuntary muscle that provides muscle tone in our intestines, blood vessels, and other structures we do not consciously control; and heart muscle, which has some characteristics of both the other types. The heart must pump against the muscle tone of the arteries and work harder if there is high pressure in the arteries. In the control of hypertension (high blood pressure), the goal is to reduce blood pressure. We would like to relax the muscle tone in the arteries without disturbing the strength of the heart muscle. Relaxing arterial muscle serves to dilate the artery, which, in turn, lowers blood pressure (similar to the way water pressure would be reduced by running the same amount of water through a much larger diameter pipe).

Amlodipine besylate is what is called a calcium channel blocker: a drug that works by blocking the calcium needed for muscle contraction. Calcium channel blockers block the calcium channels in heart muscle as well as in arterial muscle, but different calcium channel blockers favor heart muscle, and others favor arterial muscle. Amlodipine besylate is one of the calcium channel blockers that work primarily on arterial muscle. Its overall effect is to relax the arterial muscles so that they dilate and the blood pressure within them drops.

Hypertension is an important problem in both people and pets. Amlodipine besylate has become the most popular blood pressure medication for hypertensive cats. Amlodipine besylate can be used in dogs but has not been as popular as other medications.

How this Medication is Used

Amlodipine besylate is used to treat high blood pressure in cats. High blood pressure is a common consequence of numerous conditions, including kidney failure, hyperthyroidism, hypertrophic cardiomyopathy, and others.

Treatment for hypertension should be considered when a cat’s systolic blood pressure exceeds 160 mmHg. Blood pressures over 180 mmHg are considered high risk for causing organ damage. Amlodipine besylate may be given with or without food. Its peak activity occurs in 4-6 hours after oral administration in dogs. In cats, medication effects are seen in approximately four hours.

Amlodipine besylate is generally given once a day in cats. Expect your veterinarian to recommend periodic rechecks to measure blood pressure and evaluate the retinas for signs of bleeding.

If you miss a dose, do not double up on the next one. Give the medication when it is remembered and time the next dose accordingly.

Keep away from light and store at room temperature. Do not refrigerate.

Side Effects

In humans, 7.3 percent of people taking this drug report headaches, which makes headaches the most common side effect of amlodipine besylate. Of course, we do not have a good way of detecting headaches in our pets.

Infrequent side effects reported in cats include elevations in renal blood parameters, drop in blood potassium levels, lethargy, increased heart rate, and weight loss. Lethargy or appetite loss could indicate that blood pressure is too low and the medication dose needs adjustment, so report any such changes to your veterinarian. Expect periodic monitoring tests to be needed.

Gingival hyperplasia is a condition where the gums of the mouth overgrow, creating extra space for periodontal bacteria and infection. This side effect has been reported in 8.5% of dogs on amlodipine. The condition, when it occurs, resolves within six months of discontinuing the drug.

Interactions with Other Drugs

Hypotension (blood pressure dropping too low) generally does not occur with amlodipine besylate unless it is combined with another drug that drops blood pressure. Other drugs of this type might include fentanyl, diuretics such as furosemide, ACE inhibitors such as enalapril, or beta-blockers such as propranolol.

The use of cyclosporine, an immunomodulator, with amlodipine besylate is likely to increase the levels of cyclosporine.

The use of amlodipine besylate with antifungal medications such as ketoconazole or itraconazole can increase the levels of amlodipine besylate.

Clopidogrel may not be as effective as a blood thinner when used concurrently with amlodipine.

Concerns and Cautions

Pill Cutter

• Patients with liver disease remove this drug from their systems very slowly, thus necessitating changes in dose. 
  
  
• Amlodipine besylate should not be used during pregnancy.
  
  
• Amlodipine besylate has some (usually insignificant) effects on the heart. It will slightly reduce the strength of contraction. This could be significant in patients with a history of heart failure, and the drug should be used with caution in this situation.

The doses of amlodipine besylate for cats are small (usually quarter tablets) and it is difficult to cut tablets in appropriate sizes. A pill cutter can assist in accurately cutting pills or a compounding pharmacy can be used to formulate custom-made capsules or a flavored liquid.