Muscle animals compared with humans

One somewhat different, but critical, parameter in animal health sterile products compared with human health care products is the irritation and residue at the injection site. For companion animals, the pain upon injection should be minimal, and any lasting reactions that cause either pain or visual lumps to a pet will not be tolerated by many owners. For food-producing animals such as cattle, pigs, and poultry, the added challenge is that of ensuring the residue levels at the injection site have been depleted adequately by the time the animal is harvested. The formulation scientist thus may need to carefully observe the viscosity and polarity of the vehicle as they can affect residue times. Volumes injected and the route can also have an impact. Oftentimes subcutaneous administration is used instead of intramuscular administration to minimize residues in the tissue and potential damage to meat (i.e., muscle). 

Several studies have shown that the water content of human, rat and dog arteries is increased in hypertension, and this increased water content may be associated with an increased wall thickness [64, 65]. Due to the limitations in studying samples from human subjects, animal models (mainly rats) have been employed. Mallov [66] found that the aorta from hypertensive rats had more smooth muscle than normal aorta. Greenwald and Berry [67] reported increased elastin and decreased collagen content in the aorta from spontaneous hypertensive rats when compared with the normal aorta. Wolinsky [25] observed an increase in the absolute amounts of both medial elastin and collagen contents in hypertensive rats. However, the relative percentage of these elements remained essentially constant. Experimental studies [67-69] showed an increase in vessel stiffness with the development of hypertension. This increase in vessel stiffness results in a smaller vessel diameter for a given distending pressure, i.e. a decrease in the distensibility [70]. 

Phasic muscle twitches are one of the major REM phasic activities in neonates and also comprise prominent neonatal behavior. A similar feature is found not only in rats but also in other rodents and humans. The period that shows frequent muscle twitches is the first 4 weeks in the rat (4), the first 40 days in the kitten (8), and the first 8 months in the human newborn (13). In humans, this feature is more typical in the premature fetus (14). This activity appears primarily during REM sleep but also in a small portion of NREM sleep, i.e., half-activated sleep (4). The rate of muscle twitches is only 1.5/min and 0.3/min during quiet sleep compared to the rate of 7.5/min and 3/min, respectively, during REM sleep at the same age in PN 10 and PN 20 kittens (8). The number of phasic events dramatically decreases as animals mature (8,9,13,15). It is of interest to note that the dramatic reduction of phasic activities in REM sleep is associated with the increase of wakefulness. 

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