Myoplasmic Ca2+-force relationship studied with fura-2 during stimulation of rat aortic smooth muscle.
The intracellular Ca indicator fura-2 was used for simultaneous measurements of intracellular free Ca (Ca2+i) and force in arterial smooth muscle. Rat aortic medial rings were submitted to fluorometry in a geometrical arrangement resembling that of adherent cell layers. A rigid force-transducing system served to immobilize the tissue and record the developed force quasi-isometrically. Stimulation was performed with norepinephrine (NE), KCl depolarization (high K), and a nonfluorescent Ca ionophore (ionomycin) at varying extracellular Ca concentrations. The following facts were observed. NE, high K, and ionomycin increased tension along with fura-2-reported Ca2+i; under any circumstances tension was Ca2+i dependent and could be varied by manipulating Ca2+i. However, NE and high K determined a parallel increase in the effectiveness of Ca2+i in comparison with the simple ionophore, i.e., they increased the force-to-Ca2+i ratio. NE and high K produced half-maximal tension at fura-2 estimated Ca2+i of 0.10 and 0.13 microM, whereas ionomycin required 0.6 microM to achieve the same amount of force. It is inferred that Ca2+i is a determinant of vascular contraction, but some results suggest the existence of factors that sensitize the contractile machinery to Ca.
Bruschi
G
G
Instituto di Clinica Medica e Nefrologia, Università di Parma, Italy.
Bruschi
M E
ME
Regolisti
G
G
Borghetti
A
A
D000818:Animals; D001572:Benzofurans; D002118:Calcium / Q000502:physiology*; D004987:Ethers / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D005470:Fluorometry; D016257:Fura-2; D015759:Ionomycin; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D011921:Rats, Inbred WKY
D001572:Benzofurans; D004987:Ethers; D015759:Ionomycin; D011189:Potassium Chloride; D002118:Calcium; D016257:Fura-2; D009638:Norepinephrine